My relationship is technically non-monogamous, because when I met my current boyfriend I was seeing another guy who we had an established polyamorous relationship from the start (he had another girlfriend in addition to me). I'm not dating that other guy anymore and I'm totally in love with my boyfriend, so I don't utilize the whole non-monogamy thing often. Since the first boyfriend and I broke up, I haven't actually dated anyone outside of my primary relationship. I've slept with two people outside of the relationship, with my boyfriend's consent, but that is a very rare occurrence and only happens with people I have a serious friendship with (well, in the case of one of them, had. I am no longer speaking to that individual.) My boyfriend, to my knowledge, has never acted outside of our relationship--so, really, I'm the polyamorous one and he's monoamorous but he supports my desires/needs.
I think the desire for polyamory is just as much a part of your brain's wiring as your sexual orientation. Some people feel totally fulfilled in a monogamous relationship and only feel the ability to wholly love one person at a time. I, along with other polyamorous people I've spoken to, have the capacity to wholly love more than one person at a time. And there's nothing wrong with being either way. You can't force yourself to have one preference over another, obviously.
As long as anyone and everyone you are involved with is okay with you being polyamorous, there's nothing wrong with it. If you go behind anybody's backs, that's when it becomes cheating and that's wrong.