See, this is what I don't get. They obviously put this in last minute as fanservice - we all agree. And if Halsin is supposed to be this epicly handsome polyamorous guy who believes in fluid "relations" and free love and sex with everybody, how come he and Shadowheart (or literally anyone else for that matter) flirt or engage in any kind of sexual act or relationship when you solomance him? Shadowheart doesn't even talk to him or look at him. So crazy.

I found out that he suggests a relationship with you and your other romance option solely from other people and YouTube videos because this never came up while I was monomancing him.
This is so weird, start to finish. They should have never implemented the option to have three way romances with Astarion and Shadowheart. Especially considering what they both go through over the course of the story.