Now if IS by design, it would kinda make sense with those characters.

I dont remember much about the Red Prince, only that he was kinda an a***ole. Maybe they reflect that on his romance?

Shadowheart, I can see her manipulating us through her campaign (due to her background and all the Red flags she has), and at the same time be totally honest and having heart to heart moments with Tav.

Halsin, supposedly was a sex slave at some point in his live (I don't know, this is just what I've read from y'all), maybe he developed some kind of twisted sexual desire.

Astarion, seems to be the one that is less comfortable with the whole situation, but he seems to be an "evil" character, so maybe it makes sense?

Maybe the real problem is that those characters don't give you any kind of foreshadow about their preferences, but maybe they're design to be that way...

Disclaimer: I'm talking what I've gathered by reading y'all, it's very possible that I'm very wrong in this comment.