why men cheat?

I think you're generalising too much here - I've actually heard of significantly more women cheating on men than men cheating on women.To answer your question, though, it's the same reason women cheat - because they don't care about people, only about shagging as many men/women as possible. There is no justification for either gender.

Because we are force-fed the idea that monogamy is the "right" kind of relationship. The truth is that some people are satisfied with one-on-one relationships, and others are not, but society pressures us into thinking that we HAVE to be with JUST ONE person, when not all of us are satisfied with a single partner.

There are polyamorous triads and even larger groups who stay loyal to each other. It is all in the kind of relationship model that you find yourself the most comfortable with; because it is not only entirely possible to fall into deep, honest love with more than one person at one time, it is common.

If you are monogamous, good for you. Go find that one person and live happily. If you are not monogamous, then that is okay too. Go find your group and you live happily, too.

Actually from a mans perspective its all about biology. If a man is not getting what he needs or think he needs from his prospective, his body makes it hard for him to ignore the mating instinct. Its kinda hard coded into each of us that we need to further our race. From another perspective sometimes people fall out of love and want to try to keep what they have without giving up anything and it never works out ok. We men are taught growing up that showing feelings is bad and that we need to be a man about everything. So it just depends on what kind of belief you think is most right.

Lack of communication or compromise. If they can't get what they want from their partner, they seek it somewhere else (for instance, sexual gratification...sex just gets boring with the same partner, for them)--and the partner isn't even aware of what their partner is missing. Sometimes they do, but other times, nope. And I'm speaking in terms of women and men. Women probably cheat more because their better at it. Men just get caught more. All you have to do is find a guy who loves you for you and doesn't believe in cheating on their partner. I would rather have a boyfriend who leaves me for another girl than cheat on me with another girl. I'd rather know that it's over than to have it in my mind that he's there for me, not out of "Oh, I have to stay with her, but I want this other girl" guilt. But that's just me. If this doesn't answer your question, you can Google it and they actually have studies and everything on why people cheat. I've seen it on yahoo!news once and it was interesting. Got me thinking.

I saw a study that said most guys who cheat still say they are satisfied, happy, and in love in their relationship. Anyone, guys and girls included are each going to have a different reason for cheating, so I'm not saying anything is true or not, but in my experience, I've felt like once a guy is in LOVE (or thinks he is), then he really won't ever want to hurt who he's with. He may be mad and fight, but he won't want to REALLY hurt them. It may start out as a mistake, or a full-blown affair, but I feel like in the end he thinks it's better for the person he's with to just keep it from them, at least for the time being. It's definitely not a valid justification, but that may be his reasoning.