Larian I'm sure will do their own calculations on the cost of making this change vs sales. I'm pretty sure no one, or not a significant number of people, will refuse to buy the game due to a lack of sexual romantic relationship, or will rush out to buy BG3 because the game has it.
And as has been pointed out, the vampire and the alien clearly have voracious sexual appetites, and making them accept the player character's desire for an exclusive asexual romantic relationship would be bending their character design to the breaking point.
It's up to Larian to decide if satisfying this small group of people will yield the sales that will justify the cost of re-conceptualising, re-writing, and implementing this. I don't think it will be worth it.
