But that defeats your own argument. If its is about power fantasies and pleasing the player then all characters should be romanceable. If some are not because if doesn't fit their character, well thats the exact argument why they shouldn't be playersexual. It does not fit their character.
But if you're intentionally setting out to make the romances playersexual from the beginning, then you can write the characters in way that it does fit their character. Which is what Larian is apparently trying to do. Whether or not their writing holds up is another matter.
That would require making everyone bi and open to all races by default as that is the only orientation that works for player sexuality. Or you basically write most of a characters dialogue and characterization twice, and use the one appropriate for the player characters gender. But that means, as the game can't know beforehand in which character the player is interested in, that all companions are either interested in males or females which looks strange and "harem style".
Not to mention, even that approach limits the stories you can tell with the companions as they can't have important relationships in the backstory when their orientation is in flux. Unless you write multiple individual stories for them.