I'm in favor of companions having set sexualities and possibly even race/class/etc preferences. I feel that it can (if done well, and not stereotypically) add characterization to companions, making them feel more real. Additionally, it allows for backstories and companion quests that might relate specifically to their sexuality and thus resonate with certain players: e.g., Dorian's backstory and relation to his father in DAI. See @urktheturtle's point above re: telling queer narratives.
However, BG3's cast of companions is too small. For a decent chunk of the game, there are effectively only 6 companions. (Halsin doesn't count because he doesn't actually join your party until midway through Act 2, and neither does Minthara because she's locked behind the evil route).
I think the tradeoff point, where having set sexualities isn't too limiting on romance options, is at >8 companions. With 4 men and 4 women:
- 1 of each can be straight
- 1 of each can be gay
- 2 of each can be bi
This would allow for any player to romance 6 companions (3 if you refuse to be e.g, in a gay relationship). Maybe 1 or 2 fewer if certain companions have race/religious restrictions, like SH maybe not being willing to romance a cleric of Selune.
