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 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.

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.

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...