Why is it that women cheat on men so often?

An article in the London Metro yesterday, suggested that in fact, men cheat much more than women (but it also implied that that is because men, generally speaking, get more opportunity).

I have never cheated on a boyfriend, but my boyfriend of 7 years cheated on me with a 15 year old schoolgirl, he is 27 years old. Shortly afterwards I found out that I was pregnant to him, but with all the stress of us breaking up I had a miscarriage. Now I am pregnant again, this time to a guy i don't barely know.I don't know his number or name or age. I had a one night stand, thats why.

Your best bet why they cheat is because either 1) she became bored with you because you two didn't come out how she fantasized the relationship to be, or 2) she don't really know why either.