I would feel like complete and utter shit and lose myself forever. I also would not cheat on my current girlfriend no matter who the girl offered is, but that's because I truly love her and would die for her if needed.
However, I have cheated on previous girlfriends. I'm not proud of it, and it wasn't something I planned. With my second girlfriend, our fights were horrific. She'd call me worthless trash and wish I would die and stuff for ridiculous reasons, so I would stormed off to my friend's house who was having a party. Met a girl there, we both got a little drunk, and it happened. I didn't love that girl, but I still felt bad after. Not that much though.
But, like I said, I wouldn't dream of doing that to my current girlfriend.
That's quite an oversimplification. Same could be said of a lot of things, but reality is, people cheat. Just like people murder, steal, and lie, people will cheat. Their reasons are their own, but that doesn't change the facts of the matter. Human behavior is complicated like that.