In any fiction there is a part that isn't written; a part that only exists in the spectator's head. If you choose to have it as your head canon that Mars is a ruthless sex monster then that's your version of the story and it's fine.from my perspective Mars could never love or have any affection AT ALL towards Erin
I've never saw Mars as a person capable of loving anyone
At every choice and input i could have, i would choose only to use the girls as masturbators/fuck toys and nothing more, because that's what i felt it was the best course from Mars' storyline since the beginning. I hate everytime they throw affection into his persona, i don't think it fits at all, i just ignore it, pretend it didn't happen and kept on.
It's clearly been shown that Mars is not a sex monster incapable of affection; his relationship with Vi is testament enough of that. It's just that this is the one thing he's been formed to do and one he's extremely good at. But after so long ruining lives he's grown sick and tired of it.
Maybe in a future path you can push him towards more of an unfeeling tyrant but as far as the story goes, he's doing a job he's grown to dislike, because he knows the ending is never pretty and he doesn't want that.