All companions being player-sexual means by extension that all companions are, within-game, bisexual. And everyone being bisexual is unrealistic and immersion-breaking.
If you need realism or its immersion breaking for you, I have some bad news
Tell me you didn't just use the fantasy setting as an excuse to not be realistic with things that have nothing to do with fantasy... Besides, realism is still important, regardless, if you play a fantasy setting, I expect the magic in that setting to make sense in its context.