Why do men cheat?

There are a multitude of reasons as to why men cheat on their partners. Some would argue that it is a socially learnt behaviour, others would suggest that it is a lack of self control or that there is something wrong within their current relationship that drives a man into the arms of another woman! Does it relate to some gender specific behaviour that goes all the way back to the cave man? Or is it simply a case of having your cake and eating it too?