Spacesuits are compulsory

The human body is ill suited for leaving the confines of Earth. Spacesuits serve a variety of necessary functions in order for humans to operate in space like temperature regulation, providing an air supply, protection from solar radiation, etc. Those living or travelling in space long term have to be prepared to enter hard vacuum at any moment to effect repairs or modifications on the external surface of the spacecraft. Even the stripped down "casual wear" astronauts might don when not on shift would have common sense features deemed absolutely necessary like life sign monitoring, communications, distress beacons, etc.

Therefore you can go starkers if you want...under your spacesuit

1 - We're icky

There is a hygienic dimension to clothing. As mammals, especially placental mammals, we're always secreting and shedding something -- sweat, oil, hair, dead skin, and many other substances that are unnecessary and inadvisable to list.

The areas we cover up are notorious for making right messes, and even the more acceptable areas may be suspect due to sweat. This is exacerbated in low or zero-g environments due to the fact that these particles are free to detach from the body, float and get everywhere.

The very best case scenario is that the whole station will start to smell like old sweat, which probably won't be a problem for people who have been in microgravity long enough for their noses to stop smelling as well.

Worst case...well, there's lots of things you really don't want getting on your computer interfaces. Or in your food. It could even be microscopic particles, and those are just yummy.

You could bathe and shave constantly, but that's a bit of a waste of station resources and you're better off just wearing material that can catch all the gross mammalian materials and wash those in bulk.

At the very least, you'd wear something to cover the pubic area and that would narrow the problems down to sweat and stray body or head hairs. Everyone would look like they're at a topless beach party, but it would be enough for the majority of human cultures to consider "clothed".

2 - Protection

Space is pretty dangerous. Here you are floating in a tin can, far above [ gravitational reference body ]. Anything can happen; your body can be exposed to raw sunlight, gamma radiation (gamma rays from malicious sources can have devastating effects on crew), or even open vacuum. The human body doesn't really care for any of these, and so an advanced bodysuit of some kind would be preferred.

Furthermore, navigating a microgravity habitat is a constant gymnastics show. Astronauts launch and throw themselves off surfaces and try to catch bars and top themselves on other surfaces. If you miss, you're likely to collide with something or graze your body on some equipment. Something can poke your belly, scrape against your back or hit you right in the space nads. It's nice to have something to absorb at least some of that impact.

3 - Utility

Working on a space station requires versatility and high function. You'll need to carry tools and materials with you. To do this you'll need pockets, loops, velcro pads etc. It would also be highly beneficial if you were wearing some kind of harness; you can hook yourself to surfaces, brace yourself into places, and can easily go in and out of exercise equipment. The harness itself could also be a modular chassis for utility attachments, not unlike the H-harness that U.S. Marines wear.

Could you get by with just the modular harness and pocket belt? Eehhhh, maybe but the mental image at this point is getting pretty fetishistic. You can still go for it, man. It's your world, you're the creator.

4 - Comfort

I'm not gonna lie, walking around in just a birthday suit can be nice when nobody's around. But the feeling gets old, and it feels way nicer to have something soft and loose over your body. Robe, pajamas and T shirt, night gown, whatever orbits your spacecraft. Space stations aren't exactly known for being soft, warm and cuddly so wearing something to counter that can be a good idea.

5 - Fashion

Why navigate a space station in your birthday suit when there are so many other suits to choose from? Humans have always wanted to express themselves, and there's really no other universal way we do that than in what clothes we wear. It doesn't have to be anything fancy, though anything can happen. Anything from a simple T-shirt with an outdated reference from 2034 to a flashy yet functional station suit could work. How much clothing we decide to wear has varied over the millenia, yet most people from history would have agreed that people who had very little clothing were typically low status.

6 - Religious views

This one is pretty self-explanatory, isn't it?

7 - The Powers That Be say "NO"

Your astronauts of the Immediate Future may be progressively-minded, but Earth, that old world, is not. They also happen to be the ones funding your continued operation and survival. Your crew will very probably get to have some downtime where they can wear whatever they want, but in official broadcats and on receiving visitors, it's expected that they dress accordingly.

If the future conquest of space follows the example of our recent creation of cyberspace, then advertising dollars will pay for a great deal of the infrastructure that is needed. Streaming coverage of everything extraterrestrial (except private bedrooms) might be a major part of future entertainment television; like reality television only on a grander scale.

Perhaps the sponsors of the Titan station are family oriented.

Don't think anyone else has mentioned this yet:

Heat

Perhaps not a full on reason for zero tolerance as such, but what about the fact you have to heat/cool a spaceship?

Having your occupants in their own bundle of self-generated warmth means you don't have to heat the whole space as high as you would if everyone (or even one) person was nude. (Economical heating reason)

You could go as far as to say that some components on board need a cool (maybe sub zero) temperature to function properly and so low temps are a must. Perhaps as a safety feature in case of coolant containment leak everyone needs some sort of thermal protection.

Because the ship is kept at a readiness condition in space

And part of that is clothing designed to buy you precious seconds in an emergency.

Set condition one throughout the ship. Thanks to Galactica, that is the most recognizable form of naval Material Conditions. These are measures to render the ship more stable and damage resistant: which hatches to keep closed, which level of readiness the crew should have, etc.

Real material conditions in the US Navy are:

• X-ray: sitting in safe harbor


• Yoke: underway at peacetime, at risk of ordinary accident


• Zebra: more challenging or risky operations with higher risk of damage

Pleasure craft can get away with cruising around at X-ray because at sea, there are no micrometeorites. Space is far less forgiving, which forces everyone to take Material Conditions seriously.

Anyway, one aspect of Material Conditions is personal protective equipment.

In space, normal clothing is actually special clothing. First, it helps you survive decompression and remain functional long enouch to get to an emergency equipment trunk. It also helps reduce your injury in fire, and other important jobs.

You must be in in this garb at Yoke or Zebra. Yoke is the minimum possible material condition for a space station.

This became an absolutely militant requirement after a string of accidents, including the unnecessary loss of the Intrepid passenger liner with 3000 souls. Space is astonishingly good at preserving evidence. The key command personnel whose skills were needed to coordinate recovery were all found either frantically trying to put on proper clothing, rushing toward an emergency trunk, or inside an emergency trunk whose door they couldn't close because their hands stopped working. Without appropriate garb, they just ran out of time. The efforts of the rest of the crew were poorly coordinated and inadequate.

They determined essential personnel needed to have the proper clothing on at all times. This soon became a problem. First it proved impractical to distinguish "essential" from "non-essential" personnel, with lots of petitions to have ones job declared non-essential, cheating, and cries of unfairness. Second, when people were allowed to be unprepared, saving them distracted damage control crews who needed to focus on saving the ship. They should leave individuals to die, but that's asking too much. So authorities gave up, and said "everyone has to be prepared, always".

Religion.

Historically, the most common reason for prescribed garments has been religion. These garments might be obvious to all and so symbolize common membership in the religion (e.g. head coverings). Some garments are not obvious and serve as a reminder of vows to the adherent of the religion. The Mormon temple garment is one of these.

A temple garment... is a type of underwear worn by adherents of the
Latter Day Saint movement after they have taken part in the endowment
ceremony. Garments are worn both day and night and are required for
any adult who previously participated in the endowment ceremony to
enter a temple. The undergarments are viewed as a symbolic reminder of
the covenants made in temple ceremonies and are seen as a symbolic
and/or literal source of protection from the evils of the world.

Your spacefarers are the member of a new religion - or maybe an old religion or remake of an old religion. It is lonely in space, and wearing the garment reminds spacefarers of their connection to something larger - their relationship with the divine powers of their religion, and with other members of their faith.

You cannot make such a prohibition because basic sanitation and hygiene requirements require some degree of nudity at different times. See for example Skylab Shower.

In fact the requirements of space flight and relatively confined quarters essentially require that people engaged in space flights must be comfortable being unclothed at times around each other. Modesty is not worth the mass it costs and mass is the most important thing. Mass used in providing special privacy could be better used providing e.g. additional backup life support or something similar.

In an emergency do you want people worried about a ban on nudity or people who will act decisively and immediately ? Nudity should not be an issue for people in this line of work - practically speaking nothing should bother them. If they have to share crude sanitary facilities due to e.g. a breakdown, then that's what is required. No silly restrictions should get in the way of what is practical in space.

There is no policy on possession of adult materials or even engage in intimate activity, just no nudity strictly!

This actually makes no sense. Apart from anything else, no nudity isn't very reasonable when you're allowing intimate adult activity. Put crudely, if they can screw it makes no sense to even try and ban nudity.

The crew would almost certainly be chosen based on their psychological and cognitive balance as a team, with overlap of skills and the psychological ability to complement and replace each other. These people won't even notice nudity.

The idea is deeply flawed, IMO.

Perhaps teenagers and/or children are on most if not all missions. This may be a part of a high-school class, a trade-school program, or even a menial job opportunity for teenagers (even space stations might need cooks, janitors, or the like). For the sake of avoiding moral or legal issues with underage individuals being exposed to, or exposing themselves, a zero-tolerance nudity for anyone at all times might make sense.

Specific radiation answer, the primary reason:

Due to mass constraints, space vessel walls have been designed to provide enough radiation shielding just for electronics and brief human exposure. For any prolonged stays, everybody must wear a radiation protection suit. It's not very heavy, as it is designed to work with and complement the walls. But without it, you will shorten your life by decades, and the more time you are without it, the worse it gets. So taking a bath is no problem, but sleeping without a full body sleeping suit every night is going to be.

Once this primary reason is established, and having a suit is this way mandatory, all kinds of other things will get integrated into the system. For example, the air filters can be lighter when everybody is dressed all the time, as mentioned in the other answer. The suits can actually be part of the cleaning system, they are designed to sponge up dirt particles which then get removed when they're washed. Then there's the obivous monitoring of vital signs. Haptic feedback effecitvely giving extra senses, for example feeling of pressure before body actually bumps into a wall, could also be a factor, it could be a required safety measure in addition to the more obvious physical protection of clothes, not so much to protect you, but to protect the space vessel interior from wear (so again it can be made with lighter materials).

It won't be long before the space folks will feel almost as naked without their special suits, as we feel without our mobile phones today. Except in space it is positive thing, without the mobile phone dependency stigma of today. If you aren't wearing your suit, you are all kinds of rude, almost to the point of being hostile, to everybody around you. It's like walking in someones home with dirty work boots, leaving marks and dirt everywhere. On earth that just makes a mess and scratches the floor, in space it can kill you.

Others have mentioned the issues with the human body leaving dead skin, hair, sweat and the like everywhere, but there is actually a great example of this problem on earth: Nudists.

Nudists are generally expected to carry a small towel or similar to sit on, because a lack of underwear creates hygiene issues. Also as people get older they tend to leak more.

It's somewhat manageable on earth where there are a limited number of places people are likely to sit and they can be cleaned, but even so towels are often mandatory. In space, with low/zero gravity, cramped quarters...

Spacers are an independent bunch and will happily float around with parts flapping in the breeze.

Earth governments, in an attempt to maintain control over the spacers who are out of their physical control range outlaw any activity that could be part of a budding "spacer culture".

Thus, Earth mandates a number of really inefficient and downright inconvenient methods just to enforce Earth culture on the spacers.

Maybe any entertainment watched must contain ads that are used on Earth. Those ads would have nothing that the spacers could get or even use in space but they are part of the attempted enforced culture.

Also, new words would have to be approved by Earth (i.e. France).

They could have "culture ministers" who travel to make sure that no group behaviors are forming which would lead to independent thoughts.

If you've ever been to space, you'd know that there is no such thing as still air in the kinds of environments humans can function in. Without gravity and weather to ensure the constant mixing of atmospheric gases, pockets of noxious carbon dioxide would quickly envelope the head of anything that breathes. As a result, the environmental systems in micro-gravity habitations need to be constantly forcing air circulation. That ends up with a constant breeze blowing over your skin, which most people find discomforting, and with long-term consequences both to the creature's integument and to the environmental systems that collect off-cast detritus.

It's not hard to see how a constant need to cover up because of chronic environmental and health reasons could morph into a social prohibition on nudity. Of course, there's always that one guy....

Because space stations get a lot of mold (the ISS does – https://space.stackexchange.com/questions/2143/where-should-i-look-in-iss-to-find-mouldy-food), and food is never fresh, and water is recycled. This means sometimes there's nasties in the food or water.

This occasionally means people get diarrhoea.

Ever since the first station-wide outbreak of diarrhoea in zero-g, everyone agreed that keeping your pants on whenever possible was for the best.