BG3 romances operate in a state similar to the Schrödinger's Cat thought experiment. All things are possible until states are revealed.
So in my first play through Shadowheart was a faithful monogamous true love.
But, if you ignore the signs and have Halsin in the party, he's going to influence outcomes according to his well publicized sexual behaviour. If you don't want that influence seeping in, you'll need to omit him from the party and/or don't converse with him.
And frankly, for a faster play through, best to limit party members. In future play throughs I'm not going to do whole team anymore.
I like to do whole team, but Halsin brings nothing to the table that Jaheira can't too and I liek her more, so he stays mostly in camp.
"We are all stories in the end. Just make it a good one."
