This is a complicated issue. On one hand it is true that people have fixed preferences. But in the other, when you do that youvend with 1 female straight, another bisexual, one male straight and another bixesual.
And then, people would create mods so you can get the romance anyway. Making the characters "playersexual" is just opening from the begining the option for people who doesn't want to bother with mods.
Now, what would make it more inmersive/create a clear sensation of the NPC's sexuality and preferences is banter and past history. We hear Astarion flirt with Gale and, apparently, Jaezel comment on Astarion being good at sex. That makes him bi or pansexual. He should hit on the MC before the party if he is going to offer them, but him flirting with everyone is who he is.
Wyll is clearly interested in Laezel. But that should block him from being with MC. On the other hand, we know nothing about Shadowheart excepto that she didn't liked Gale whrn they met. Gale seems straight, because Mystra and Shadowheart are both women. For him to be with a man, a conversation ir comment needs to happen, either acknowledging that MC is the first man he is attracted to or mentioning past lovers. The point is that you can make the full group bisexual or pansexual, but it works better if it is woven in the story