I mean sure, the power fantasy aspect may not be the draw for a lot of people, but nearly all of these RPGs have a power fantasy at the core of their story. They're almost all about the PC being, or becoming, very powerful, and doing very powerful things like fighting god-like beings, saving the world, or ruling a kingdom, or the like.
I don't think that one's gender preference is necessarily part of one's personality. I don't think the Jane who wants to fuck both Bob and Mary has to be any different, personality-wise, than the Jane who only wants to fuck Bob. I mean she COULD be different, sure. Some people make their sexuality a big part of their identity and like to express themselves about it. But not everyone does. There are plenty of people out there quietly going about their lives being gay, and their families/coworkers/friends don't even know it.
I definitely agree with you that EVERY character does not need to be romanceable. I don't think Jubilost or Harrim are romanceable? And that's fine. Romances should only be written for the characters that would be likely to engage in one. It's fine for some of the characters to have no romance option. But if they're going to write a whole romance for a character, it's not much extra effort to make that romance work fine no matter what gender the PC is.
In the case of strong feelings about certain races, there I agree with you as well. Shadowheart opening up to a Githyanki that quickly (or at all) is inappropriate and DOES detract from her characterization.
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.