I haven't gotten there yet. My thought is, why is only Halsin okay? What if I want say Shadowheart and Karlach?
The theory here is, Karlach is going to either die or go to Avernus, and nobody is surviving a pregnancy in Avernus without a diabolical contract or 3. So my PC and Shadowheart might as well give Karlach all the love we can on her way out.
I imagine it's probably because having two females at the same time is some sort of "male gaze" issue.