If you would cheat on him that easily I'd argue you're with him for convenience or because you settled. In any case, I'd hate distrusting my significant other to that point. I would really not be able to have a relationship like that. YMMV though.
But it really has nothing to do with settling; some people are just not monogamous by nature. I'm sure you can understand, since people sometimes make that generalization about the entire male sex :).
Whatever suits you guys though, if you guys are happy then more power to you. I'd would just find it incredibly difficult.