Well said, Mizukun! I can only add that Halsin's behaviour is a sudden restriction of sexuality where none was before. Our companions are not straight, gay or anything else - they were made player-sexual for inclusion reasons. Stating "he is this way, deal with it" is kind of contradictory to the theme. Wyll flirts exclusively with girls in banters, while we only meet Astarion's male lovers. Yet, Wyll can perfectly marry a guy Tav and Astarion confesses his love to the girl Tav. It's all up to the player.

I see no reason for Halsin not to do the same thing, depending on what the player wants.

(Has to say it again about the "lore of wood elves". Their polygamy nature is not a DnD rule or lore. It was mentioned in one DnD-related novel. One. Once. Larian can absolutely make it part of their game but since it does not apply to anyone else in the game world, it does not look like a rule there either. More like Halsin's personal nature.)