Why do people cheat?

I know men and women can both be guilty of cheating, but why do so many men tend to stray during their 'midlife' years?

I spoke with a guy I go to school with the other day. He is 51 years old and looking for a girlfriend, even though he's married. He said he was looking for someone new because he was "bored" in his current relationship. I suggested he tell his wife how he feels but he said his actions were working better than his words ever could. He said he and his wife have not been intimate in over a year. He added that he told his wife in the beginning that he wouldn't be faithful, but she chose to marry him anyway. According to him, he has remained faithful during the 15 years they've been married, but he's tired of it.

This scares the hell out of me. The National Opinion Research Center at the University of Chicago published a survey in 2007 claiming that 12% of women cheat while 22% of men cheat. Dr. Gail Saltz, a psychiatrist with the New York Presbyterian Hospital believes the range for men is actually around 22-70% because many men do not admit they've been unfaithful. That's too much cheating! What happened to being faithful, honest, respectful, and loyal? Now I know it sounds like I'm bashing the guys, but I'm not. I know women cheat too, but I'm sticking with men for now only because of the disturbing conversation I mentioned in the beginning.

Dr. Saltz also said that men cheat because they are looking for 'new sex' and 'variety,' not because the mistress is more attractive or younger. I don't really care what Dr. Saltz says! Does this mean I will one day sit and wonder if my man is out sleeping around because I've gotten old or boring? I hope not! I'd like to think I will be able to trust the man I'm with always and forever, but statistics say that's not always the case.

What does it take to keep a man faithful? I'm hoping an honest, open relationship is enough, but what do I know? Any guy input on this would be greatly appreciated.