I really think that monogamy is a pretty inhuman concept that leads to lots of conflict within us. I personally think that sex and love are two different things, I can be in a good relationship with a girl and **** lots of other girls without that affecting my emotions for that one girl.
On the other side, we live in a society with certain values and monogany is not the ony one which is unnatural and ****s us up, so I would say no, you cannot love a girl and cheat on her because that would really hurt her, and that is universally bad, indipendant of how you live.