Cheating on a woman is the least masculine thing you can do. Honesty and self-control are defining characteristics in my view of masculinity.
Was in a relationship for 4 years. I knew it wasn't working, so I faced the truth and broke up with her, despite feeling deeply attached to her. It was one of the most painful things I've ever done, but now I know that I don't want a committed relationship, but I do want healthy relationships with quality women and I'm honest about that.
Do whatever is right for you, but for the sake of others, don't be a liar or cheat.
Do you not think it's a natural instinct for men to "cheat"? Cheating is a relatively modern concept. We've evolved to a point where society encourages monogamy. Why does a modern way of thinking define what is or isn't masculine? I'm not saying that morally it's correct, but simply that labelling as not masculine is ambiguous at best. We all have our own definitions.
My understanding of genetics and human instincts is gained from experience and reading books like The Selfish Gene by Richard Dawkins etc. Our genetics effectively hard wire us to reproduce, everything else we do is emotional fulfilment. Our conciousness allows us to make decisions based on morals and ideals. Anyone who's been in a relationship and flirted with another woman would have felt the cognitive dissonance caused by the conflicting thoughts that you should be faithful to your partner, as to not cause emotional distress, but then you have a basic instinct of wanting to.. mate with another attractive woman.
I agree though that if you are inclined to sleep around then don't do it while in a relationship. What goes around comes around..