As long as you can disable nudity, which you can in BG3, and it is completely optional to do that I can't really see a problem with it. It would be good if there where som kind parental controls connected to steam or something.
I think evolving characters and relationships with your companions is pretty realistic and adds a lot of depth to game and that is something that is missing in a lot of similar games.
Personally I think it is educational rather harmful for teenagers playing the game that you are able to build relationships that eventually that develops into love and then maybe even consensual sex. Especially compared to all the pornography that exists on the internet. This is way better.
