It takes two to tango. Unless they go off with another bloke, then there is another girl willing to take them. Ask yourself why they might find another woman more interesting/attractive. What I'm trying to say is, why should they stay with one person if they find they like another one more.
No offence to the nice blokes out there, it just appears to me that over 50% are unfaithful. It was just something i have always wondered. If a bloke cannot be faithful surely he shouldn't be in the relationship. I do also not understand girls who are willing to pull a bloke that has a girlfriend. All the nice blokes out there should get a prize for being the wonderful chaps that they are...