Why wear swimsuits/underwear in a sauna?

In all the saunas I've been to in public spas and sport centres in the UK it's swimsuits that are mandatory and not nudity. Private saunas are a different matter but they were alternating between sauna and swimming in the film.

There will always be women in rubber flirting with me.

Well I live in New York and everyone just wears a towel.

But i mean these are obviously a group of friends (both genders) and i feel if in that situation we would all wear our underwear too.

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You sit on a towel for reasons of hygiene, so your sweat doesn't soak in the wood and the next person sits in it. You're supposed to sit on the towel and have your legs on it when you lie or put your feet on the lower bench.

Eh..I don't think it's due to any rating system. There's plenty of nudity in US films.
It was being more realistic.
2 college aged couples double dating are not all that likely to be baring it all that casually unless they have hopes of a foursome.
Besides, Jenna had just met Ian. She hadn't kissed or slept with him and was kind of the shy sort. She was even modest at first about being seen in her swim suit.
It's not a prejudice about nudity...it's just unlikely these kids would've done it under these particular circumstance.
Not even counting the fact of guys that age being so protectively possessive about "showing off or sharing" their lady beyond a bikini...unless they're wanting group sex it's not something they usually go for..
Nothing wrong with wearing a swimsuit. Really.

This is a bit long so if you choose not to read, that is fine.
I don't think prejudice is the correct word for why many 'US people' don't go naked in a sauna; I think a more appropriate phrase would be that many of us in the US have a more modest point of view when it comes to nudity than the UK does (never been there so I will take your word on their view of saunas nudity).
Like others have stated, I and many of my friends don't want to sit naked in a spot that others have been naked and unsure of their personal hygiene.
You say its unhealthy and unpractical; usually the people are wearing swimsuits and when they get out of the sauna they shower then change, so we are exposed to the danger for an hour or so. Most people are in underwear while working outside or exercising for hours and I have never heard of any issues arise; I guess UTIs are possible but this is not how I hear of people getting them. Unpractical is a person view so I can't argue that one because its like arguing religion, nobody is correct.
Lastly, when my friends and I would exercise at different health clubs or during a vacation and we would head to the sauna at the end right before the shower, the only people that would get naked were the truly fat (at least 350lbs, 35% body fat) and then proceed to stretch in their while naked while every one of us that was in shape went in with our swimsuits on. That is my personal view of nudity in saunas; it may not be correct but fat, gross and stretching is what I assume when someone says people are naked in sauna.

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What you say in public is what you want people to hear; what you say in private is who you are.

Different cultures have different attitudes toward public nudity and saunas. But clearly this was a decision based on ratings, because in the U.S. and many other places, visible genitalia would get you the kiss-of-commercial-death NC-17 if not banned outright. And having naked characters' offending body parts just happen to be off-camera for 90 minutes would be ridiculous. But culturally, with Americans, it's all over the map. Many, many people--I'd say most of mainstream society--are too uncomfortable with nudity to take a sauna or go skinny dipping even with close friends (unless they're of the same gender). On the other hand, I live in California, have gone to plenty of "clothing optional" hot spring facilities (where basically no one "opts" to wear swimsuits), and it always seems utterly wrong to me on those rare occasions when I find myself in a hot tub or sauna where everyone is wearing clothes. I mean, to me, that's as silly as wearing clothes in the shower.

I think it really depends on the people. There are plenty of places where people are normally nude in a sauna but even ignoring movie ratings, wearing underwear/swimwear in a sauna is hardly uncommon as even the OP notes, especially in the US (or Canada where I'm from). It's entirely possible that these characters have always been just to saunas where a swimsuit was mandatory (particularly in mixed gender cases) and it may not have even occurred to them to go nude. Plus we see early on that two of the characters (Jenna and Michael) met for the first time.

In short neither the characters wearing something nor the characters being nude strike me as unrealistic. Just the characters' personal preference, what's normal for them.