Hard disagree
First of all sexuality is not a character trait.

When games have strictly gay characters they are usually

1. Not relevant to the main story
2. Very flamboyant
3. Morally bankrupt
4. Their romance quests are about beeing gay

When games make companions "herosexual" it allows everyone to pick and chose, there is no drawback for anyone.
Also characters are not written with stereotypes in mind and their romance quests can for once be about more then beeing sad about how gay you are.
I think in bg3 the characters will not / can not end up as "vessels for your MCs attraction" since their attraction is based on your actions.
Yes its not realistic to have all companions be pansexual but its also a game about wizards and dragons. Not to mention you wont even notice them
beeing pan unless you play through it again as a different gender.
And again why or how do you expect a romance to change if the gender of those participating changes?