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.
Firstly, that supports my point about Halsin (and Astarion) being the star companions. Secondly, it is bad writing or writing to support a particular agenda if an NPC is the driver and not the PC. Thirdly, Knowing it is better to leave out Halsin is due to hindsight.
Schrödinger's Cat? It more like Hobson's choice. https://www.merriam-webster.com/dictionary/Hobson's%20choice
I find it interesting that there are a lot fewer posts about Astarion than Halsin or several other NPCs.
I killed Astarion at the start when he pulled a knife on me. I rescued Haslin in the goblin camp mainly due to the Grove quest and the 'find the Nightsong' quest. I never had any intention of having him my party and didn't want him in my camp.
