I think that's a lie designed to make men who do cheat in their minds feel better about it. - Edit 1
Before modification by Rebekah at 30/07/2010 12:42:04 AM
No man is mentally monogamous. I guarantee you every man cheats in his head. Every day. But that's why the power of choice is so wonderful. We have free will baby
Agreed.
Thankfully it's still a long way from "in the head" to actually doing it 
Not every man finds the idea of sleeping with another woman attractive. Thus they don't think about it.
It seems to me that the "every man cheat in his head" thing is a total cop out to prevent feelings of guilt.
That's what I think, anyway.