I've just been replaying the game (with the new honor mode) and once again, the characters come across as way too needy and wanting to jump your bones way too quickly. There is little 'buildup - I should have to go looking for that, not just be nice to people. BG3 seems to equate 'being nice' to " I have feelings for you'. Maybe they should have hired a relationship advisor and not just someone for their sex scenes....
From my experience that depends strongly on life experience.
I had a girlfriend with whom I spent lots of time and it took 1.5 years to the first kiss (and then only weeks to the breakup :-/). With another woman I met we’d have been in bed after three days — if I hadn’t declined. And yes, I’m married today.
But studies say, "couples moved rapidly into sexual relationships. Over one third reported having sex within 1 month of the start of their relationships." (Sassler et al 2012) and regardless of how quickly you start to have sex, your relationship is better when you don’t move together too quickly: "the association between relationship tempo and relationship quality is largely driven by entry into cohabitation" — though "women who take longer to become sexually intimate with their partners report significantly greater levels of sexual satisfaction".
So I’d say BG3 is well within the realms of plausibility, especially since it’s an extremely unusual, exciting, frightening and intimate situation for all the characters.
