Really I don't know. There isn't an end all, be all, answer. Men cheat for so many different reasons and every situation is different. I will however detail a few situations I've witnessed/experienced.
1. Curiosity- When I was 21 I cheated on my girlfriend at the time. She didn't do anything to deserve it I just wanted to know what it felt like. I had been reading a book by Michael Baisden on cheating and it was fascinating. Not to mention growing up I never saw a single faithful relationship so maybe it was inevitable. But I did learn some valuable lesssons: the pain you cause your partner is never worth it. And it hurts 10 times worse when it happens to you. It's like I always say: Karma is a hoe with herpes, you might not see it at first but she's gonna eff up your life.
2. Relationship problems- This to me is a cop out. You're unhappy in your relationship so you go find someone else on the side. I did this once and like we saw in Tiger's case the sidechick almost always snitches. Maybe I should've had tighter game back then but I was young! Tiger's a grown ass man! The relationship problems could be a nagging partner, a crazy partner, a partner that isn't fulfilling your needs etc. but the man is jus looking for an escape route from
3. Lust- Some men just can't control themselves. Shit happens. Let's be honest, there are lots of thirsty chicks out there. It always seems like when a girl finds out you have a woman she steps up the level of her flirt game. Even the strongest most committed man might get weak with enough temptation. You just better hope your man is wise enough to remove himself from the situation before things get out of hand. In recent news it seems like the new fad for cheating men is claiming they have a sex addiction. Lust isn't anything new. Shoot, Jesus was talking about lusting in your mind 2000+ years ago. Most dudes don't have a sex addiction they just like getting up in that wetty-wet-wet. Ain't nothing wrong with that I suppose, as long as you're not in a relationship lol. (of course fornication is Biblically wrong, but not errbody believes in the Good Book and God)
4. They don't take pre-marital relationships seriously- Lots of men believe that if you're not married you're single and they behave accordingly. I agree with the general sentiment but I still believe you can have committed non-marital relationships that should require monogamy and fidelity. I remember growing up I was always encouraged to have multiple women as long as I wasn't married. The funny thing is that these same people that told me this stuff cheated in their marriages. You're not gonna just flip a switch and get your act together. It takes practice.
5. Their women enable them- Frankly, there are some women who just don't give a shit. They don't care if their man cheats on them as long as they get to be the main chick. Call it low self-esteem, progressive thinking, or craziness, they are okay with it. They keep quiet; and assuming the man doesn't bring home anything he didn't leave with, whether that be diseases or babies, they're cool. We all have are levels of tolerance maybe they're more tolerant than the rest of us. Frankly, I got too much pride to share my woman with another dude (assuming I know of course). If she needs to step out then she needs to choose. "Mr. Pretty Tony you know the rules of the game. I mean yo bitch just chose me. We can settle this like you got some class; or we can get into some gangsta sh!t"
This list isn't by any means exhaustive because like I said before there are just too many potential reasons/scenarios. A few people had asked me to post on this so here ya go. Here's my 2 cents.
P.S. I'd imagine these reasons can translate across genders and sexualities