People are not only playing RPGs for power fantasies. I would even say those people are the minority. Story and immersion are a big part of classig RPGs in the style of Baldurs Gate and its successors like Planescape or Kingmaker. And both story and immersion get damaged by playersexual companions. Ones gender and in fantasy worlds racial preferences are part of ones personality. Yet if all characters have to be playersexual you can't touch those parts of the personality, meaning you can only write bland cookie cutter characters which are open to everything. How do you write someone like Jubilost (Kingmaker) who is full of himself and thinks everyone is beneath him when he will have sex with low int barbarians (who he does not respect at all) just because thats what the player plays? Or Harrim (Kingmaker again) who is a disillusioned priest waiting for the universe, or at least his own life, to end. Him having a romance would completely be out of character, so why should he have one. Even Bi characters who are by default player sexual are more memorable when being bi is part of their character and not just happens by coincidence (Octiavia and Regongar, last Kingmaker reference I promise).
And thats just gender. There are also racial preferences. Just take Shadowheart. She hates Githyanki, but will do you after a few days of knowing you because you are the player. This is just eye rollingly bad and devalues her personality.
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.