I can see Zack in a monogamous relationship, but, like you said, only by circumstances. If left to his own devices and given accepting lovers, he'd have a harem to put any shoujo comic to shame. And a much happier one since, y'know, they'd all be getting laid.

(Yeah, a little non-con-ish, but I happen to like non-con and dub-con if the author doesn't try to pretend that nothing skeezy is happening.)