Sounds like the issue is that you feel sexually attracted to her at a purely base and instinctual level and you don't want to feel that way towards someone you consider as a daughter, and it's easier for you to try to have her fix the problem than for you to process your own lizard brain response to an attractive woman being near you.

There is nothing inherently wrong with feeling the way you're feeling, it just means that like most people you are not actually as noble and pure as you would prefer to be.

If you can process that whole shebang then it might make it easier for you to actually step into the role of being closer to your own personal ideal of pure.

I would start with taking a deep breath, holding it for the count of three, and then exhaling it and noting how you feel, what you feel, and how that makes you feel, and then accepting that without being influenced by it.

Sexual urges are normal. Not being in control of them is not normal.

On the other hand, it would definitely be inappropriate for her to walk around entirely in the nude

Now I'm going to get downvoted to oblivion for saying this, but just hear me out. Why would she be inappropriate even if she did that? How does the act of being nude at home harm anyone? There is literally not a single explanation as to why nudity must be looked down upon like so.

The only reason why nudity is perceived to be inappropriate is due to cultural/religious reasons. I believe it is due to this misunderstanding that being naked equals having sex. This however is obviously not the case. Scandinavian culture is much more libertarian in this regard. Look up their "saunas". Compare this to Middle Eastern culture for instance, where skin isn't appreciated much.

When we look at your situation from this point of view, should an individual really have any right to dictate another adult individual about their choice of clothing? I believe the only reason why the revealing clothes are making you (or anyone for that matter) uncomfortable is due to cultural/religious influences. Shake that aside, and this situation doesn't even remotely become special or weird.

Just say nothing, but start wearing your bdsm outfit.

As a woman who is attracted to other women it has never been a problem for me and my friends to get dressed together or hang out together wearing far less than what you are describing. The only reason I can think of for it to be uncomfortable is if someone is viewing someone else with desire. If that is not the reason, what is? If it is the reason stop doing that and you won't be uncomfortable.

There is no universal "line". Every family has its own implicit rules about clothing and nudity, which vary from situation to situation.

If she doesn't question the rules acquired on her mother's side (your SO doesn't seem to be worried by her clothes), it's because she sees you as an adult in the family, i.e. someone she trusts and who won't sexualize her.

If you feel uncomfortable, I think you should work on that on your side. I think these days, it must be precious for a young adult to have a safe place, where she can dress without being judged or sexualized. You shouldn't risk taking that away from her.

Personal opinion: casual nudity should be more prevalent and in the longer term makes it easier to handle nudity without associating it with sexuality.

If it's really warm, you can't really blame her. It's just a brief period during the year when she'll be like this, so what's the harm? Not sure where you live, but I'd wear nothing but my underwear where I am right now.

I'd say the best course of action would be to say nothing and just ignore it. If my step father would say something like that to me, I'd feel a bit uncomfortable. It's up to you though, you know your family better than we do.

It's good that you've acknowledged your biases. I think everyone's voice here has been pretty uninanimous: her attire isn't inherently an issue.

If it makes you uncomfortable you're fully entitled to voice that, but you should be open to the fact that the answer will probably be "that's a bit of a you problem".

On the other other hand, it would definitely be inappropriate for her to walk around entirely in the nude, so there's got to be a line somewhere.

Question here is - would that really be inappropriate, or is it also just the way you feel?

Where I'm from it's not seen as a problem to see family members nude - my wife coming from a different culture was a bit surprised when she bumped into my dad on the way to the shower the first time, but got used to it quickly.

Especially around the pool - assuming it is shielded from the street - I wouldn't see any issues about not wearing clothes (and wouldn't bother myself).

I don't think any amount of clothing, or lack thereof, should be treated as inherently sexual/inappropriate. She should be able to wear what makes her comfortable, especially at home.

Not sure if I can give you specific advice, but I would say that your statement that her being naked around you is also a cultural "line" that's not written in stone. Now I'm assuming that you're from the US, I'm from a European country where many people don't have that line. While I don't have any step family of my own, I can say that I've seen and been around many of my in-laws naked without there being any issue. I also grew up in a family where we all saw each other naked all the time, and my wife have a stepfather who's seen her naked since she and her mom moved in with him when she was about 14. That being said, even in my country I know friends from other families where that would be considered weird, so I'm not trying to say your thinking is inherently wrong. Maybe you should start by discussing this with your partner to get their view on it?

I'd talk to your SO and have them have that conversation with them.

It's okay to have boundaries in your house regardless of whether they are an adult or not.

Seems like my opinion is an unpopular one here but here goes:

If the tables were turned and it was an adult revealing too much of their body to a child and making the child uncomfortable it would be inappropriate.

If someone goes out in public naked under a trench coat and exposes themselves to people who don’t want to see their genitals it’s sexual assault.

I realize different countries have different social norms but just because a scandanavian country is ok with nudity doesn’t mean that the rest of the world is or has to be. You have your own social norms based on where you live and at least in the US, certain body parts make the majority uncomfortable when visible. I don’t see this as right or wrong, just local customs.

If social norms were to change to accept nudity then that would change the situation. Some of these comments downplay the fact that it makes you uncomfortable just because nudity is accepted in a scandanavian country. Which is like saying: “In middle eastern countries woman must be fully covered at all times so you should live by that custom”, which is the opposite side of the same spectrum of acceptable nudity.

It’s my opinion that we all have to coexist and that means taking other peoples feeling into consideration to some degree. Even if those feelings are formed from social norms that change depending on where you are.

To answer your question, I don’t think it is good to talk directly to the daughter and tell her to change the way she dresses but I would let her mother know it makes me uncomfortable and try to find a compromise.

If you talk about a lot of topics, and you've got a degree in communication, wha the heck don't you talk to her about that? That it makes you uncomfortable, etc? WHY?

Lol actually sounds something straight outta nubile films

i mean it doesn't hurt to simply let them know it makes you uncomfortable and gently ask if they wouldn't mind taking it into account, and if she doesn't volountarily change you're simply back at square one.

I don't have degrees in communication and sex therapy, but here's an unproven idea:

Rather than asking her to change, have you considered showing signs of being slightly uncomfortable, maybe by visibly shielding your eyes or looking away, for example? (It could be done in a half-joking manner.) That way, you're calling her attention to it without imposing restrictions or judgment or putting her on the spot, and she can decide on her own terms whether it might be a good idea to cover up a bit.

Since you've already established open communication and trust, perhaps she would initiate a conversation about it. Or if she knows your upbringing, perhaps she would quietly adjust her behavior to avoid making you feel embarrassed.

It's ok to feel uncomfortable and it's your right to talk with her and explain how you feel. In my opinion, she should be free to wear whatever she wants, when it doesn't bother others, but since she is not alone it's understandable that there should be some compromise.

I agree with other commenters saying basically it's normal, but she should be allowed to live her life. You are not your first thought, your lizard brain will react to a stimulus with an immediate thought, but what make you be you is how you react to that thought. Do you lash out, do you act, do you beat yourself up, or do you say no thank you to that thought. Those are the things that matter.

I think there is something to be said for boundaries. Everyone she meets in life is going to have different boundaries, including future roomies. She would do well to be mindful of what people in her closer vicinity are and aren't comfortable with. My fiance once had a roommate who really liked being nude, and my fiance was not okay with it. We are also learning about the asexual spectrum in our house as we realized that my fiance is Gray-sexual. So we are navigating new understandings of boundaries, consent, and desire. It might be a good idea for you to do some contemporary research on these things, and after gaining a robust understanding, sharing that knowledge with your family could be beneficial.

But if you charge into that and follow it up with "so you should wear more clothes" then you'll have lost the pot. Share that knowledge and let them run with it instead