"How many guys cheat on their girlfriends? I do - it's called insurance
To me it all boils down to economics.
Unless you are an alpha male, male sexuality is inherently worthless. However, female sexuality, of any variety, is valuable.
Said differently, an average male has a very difficult time getting sex, but an average female does not. Average men will greatly lower their standards just for sexual gratification - but average women do not have to. They get a multitude of choices, and simply pick the best male out of their current buffet.
So because of this reality, for emotional insurance, when I do get an opportunity to sleep with another woman other than my girlfriend - I do it. Because I know if my relationship with my girlfriend splits up - she'll easily find another guy to fuck within the next week, while I won't (unless I'm just lucky). So to insure against this, I have sex with other women throughout our relationship, so I have that to absorb the blow of her instantly picking someone to fuck so soon after our potential breakup.
So now many will probably ask - what if you don't ever break up, and you decide you love her? Well, my economical argument still stands. From an economical standpoint, my male sexuality is worthless, while her female sexuality is valuable. In most relationships, the male buys a greater number of gifts, pays for a greater number of dates, invests more labor (fixing things around the house, making more money than her, etc.). So a males value to the relationship is his resources. Because it's very clear what each partner brings to the relationship (male = resources, female = sexuality), it would be much more damaging if the FEMALE cheated than the male. When a male cheats, he isn't giving away money or labor, he's just giving away his worthless sexuality. When a female cheats, she is giving away the only value she offers to the relationship."