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.
Lets talk realism then. Larian studios is trying to reach a broad and diverse customer base and has 0 intention of enforcing sexual regulations on people in this day and age. Are you immersed again?
What is the problem you are solving? Does your proposed change solve the problem? Is your change feasible? What else will be affected by your change? Will your change impact revenue? Does your change align with the goals and strategies of the organizations (Larian, WotC)?
