My response? Quit meta-gaming. If you can’t do that simply restrain yourself and don’t romance the same character with both genders. Unless the companion in question makes his or her sexual preferences explicit, your player character does not know what they are. If you think that person should be straight don’t initiate a homosexual relationship and vice versa.
Then there’s the sub-argument that BioWare’s games are too player-centric in general and the move to variable sexual preferences is a further step in the wrong direction. All I can say in response to that is to ask why you’ve chosen to play an RPG if you don’t want to influence the game universe. Is that not what they’re for? Go play Halo if you want an exciting game with a good story with characters you can’t change.