The last two posts, jesus. I mean I don't like that she agrees to it either and I'd like it to be rewritten but at the same time this is optional. I went to the brothel, talked to the Drow twins, told them no thanks, and that was the end of it, Shadowheart didn't say a word. If an exclusive relationship is what you were looking for I don't know why you're trying to sleep with the Drow twins in the first place. My take on it is it's something she'll agree to because it's what her significant other wants but not something she's looking for since she never asks you to do it herself and if you want to have an exclusive, monogamous relationship together she's happy to do that.
Like I wrote earlier. Imagine your partner taking you to a sex club and initiating an exchange of sexual favours with some sex workers and asking if you want to join. So you agree that if that's what they really want, you are down with getting a bit freaky and exploring some fantasies with other people (and before someone chimes in that this happens out of the blue, do note that this takes several conscious and very clear choices in a dialogue tree to get to). Then they turn that against you, judging you and giving you a guilt trip of shame about it. They are obviously the problem if they pull this sort of emotional entrapment stunt on you, not you!
Now I have personal boundaries around anything that relates to traficking, and for that reason I won't ever make the choice in a game to buy sexual favours. I know this is a trigger for me, so as soon as the option appeared I declined and moved on (I don't have any issues that it exists in a setting, but engaging with it feels very wrong to me). I didn't continue exploring it, out of curiosity or some other strange psychological compulsion, just so I could get triggered and plunge myself into some deep black hole of horrible feelings.