Im under the impression this isn't meant to be the final version of this, but admittedly its only cuz of how ham-fisted and out of nowhere it is. No one could have looked at this and said "Yeah, this makes sense for this group who has known each other less then a week"
Githyanki girl especially, i didn't get her romance option but even the refusal sounded all kinds of off for her
The only companion for which I think a one night stand makes sense is Astarion (i mean, the dude probably wasn't allowed to have sex for over 200 years - you would be horny too).
Completely agree with this, I had talked to him first and found it pretty natural only to then be wierded out by everyone else wanting it too