Yeah, I mean, I’m a queer polyamorous woman so very much in a different world (perhaps within your “much less likely” category :p). I do see where you’re coming from; I guess there is a difference between being honest if asked something like “do you want to fuck my sister?” (maybe stupid question to ask if a yes would be hurtful, but imo very important to not lie), and your original example of asking carelessly and unprompted if it would be okay to do so, which does raise a lot of questions as you say.

I do think it’s fundamentally unrealistic even within monogamy to expect your partner to never be interested in anyone else, but probably realistic to expect them not to take steps towards being with someone else without a very careful conversation first. And that conversation is much better done before there is any particular person in mind, otherwise it can come across as “I want to date this other person more than you, but also I like the security of this relationship” etc etc.

then think its OK to ask their wife how they would feel about a three-way with her sister.

I can never understand how relationships are supposed to function at all if one person would take issue with honest communication. That’s not a relationship I would want to be in. But idk, maybe I just don’t understand straight people.

I’m in my mid 20s and a woman and I found out recently that the skinny jeans I’ve been wearing for the last decade have supposedly been uncool the whole time. I don’t care. Style is personal, and I like them.