I don't think it's a bad idea for companions to have their own preferences. Making them completely playersexual is... I don't know... boring? Not interesting? I remember some people were confused when in Dragon Age 2 all companions were bisexual, some did not like it. It's just... people in real life are not all bi/pansexual and do not prefer both mono and poly relashionships (yes, I know, the games shows us not real world but still). Also, there are such things as head canon, fanfiction and stuff where anyone can use their fantasy in case if they cannot romance a particular character (or if they like NPCs).
I think turning all companions to be open to poly relashionships (to have such option if player decides) will need much more resourses than solving problem of two the most problematic ones.

Originally Posted by AmayaTenjo
He is not my side piece. He is my beloved.
This! approvegauntlet