It may well be that someone has a reasonable expectation of monogamy, but that's entirely irrelevant. If you don't like the way your partner acts in a relationship, you don't hit them, you talk about it and possibly break it off.
Whether or not monogamy is a gift or an entitlement depends entirely on your culture / value system. I think that abusing someone's trust is, well, abusive.
Couples should figure out where they stand on that, before they resort to kicking.