The association of "evil" happens because that's where their experience and trauma comes from but still doesn't make sense for it to be that way with many inconsistencies and wish for it to be changed on their "good" paths (Selune/Spawn). Maybe you a Mod can put in the good word for this to be changed and altered.
I'm afraid that both as a mod and as an individual I am uncomfortable with the implication that an openness to poly or open relations per se is equated with either evil or trauma, and am very much in favour of giving gamers the opportunity to roleplay more positive open relationships with Selune Shadowheart and Spawn Astarion if that is what they wish. I would strongly disagree with the suggestion that the option is removed, and think doing so would send a worryingly negative message about Larian's attitude to non-monogamy.
Which isn't to say that I think that all the writing is consistent and couldn't be improved, and as with many other places in the game I'm sure tweaks could help make it better. Though I'll admit this is not a priority area for me.