I mean, you do already somewhat choose your protagonist's sexuality through your choices of who to romance. There doesn't need to be a specific flag that you choose at character creation.
I think a better option would be to add more dialogue options to when companions proposition you if you accept/reject them, including of course a "none of your business why I don't want to sleep with you" option.
What about bi option? I am all for subtlety, but we do choose our gender anyway and adding sexuality in a game, where it matters somewhat, these days should be just a logical next step, no? The option "no romanses" can be added too, so people indifferent or disliking this sort of things can play in peace as well.
Do not think it will be implemented any time soon in any game, but one can still hope.
