I've always been on the fence in regards to companion sexuality. Dragon Age had characters that had specific sexual orientations, or even racial preferences, and I know that the community were mixed about it. Other games have companions being "playersexual" where they will always flirt or romance the PC regardless of their various attributes.
I'd like to include the the idea of evil/good/law/chaos attributes amongst things like appearance, race, and gender too. I know your actions have a lot of influence in your companion approval level, but in other games (and, to a certain extent, this one too) its entirely possible to game the system and get max approval with companions despite your actions. I think it's a little odd for your good companion to try and romance the evil PC unless you're trying for some kind of redemption through love story, or you're taking the stance that your evil PC is fooling them into believing that you're a good person.
On the one hand, if done well it can create variation and add to overall feel of a character. But on the other hand, having playersexual companions give players more choice and agency to make their own story without the player having to cater too hard to the needs of the romance plot.
