Ok just to be clear what happened, there was an orgy where someone invited a third male person who was then more or less jumped by a few horny women (not the sisters, though they get freaky with quite a few women) while MC was busy with a few others. That person got almost raped, though he did not resist and seemed not too unhappy. He was absolutely not the active party there.
You later had the option as MC to either have sex with someone who might have had sex with that guy or not. Only thing that might be bit cheating there is that Haley, the nurse, could be depending on your choice one of the people that did it with the ex-virgin boy.
Now I also think a warning would have been fair since the start of the game did not really hint at sharing so much and that scene together with a few lines tells us as players that sharing and swinging is well possible in this game and likely even a theme, especially with other women (I know most people care way less about that as with males, but it is still sharing.). There are also two hints of bad guy storylines of which we have no idea right now if those might also involve the girls sexually. If they would, then a warning for a planned NTR tag would be wise in dev notes.
Considering the lesbianism question, there is quite a lot of that but also quite a bit of MC getting his dick wet, I would guess it is about 50-50 until now if do not count that orgy. Maybe a bit more as I prefer as well, but that is why the skip function exists.