You know, they say a man thinks about sex every 5 minutes or so. I heard all sorts of figures but I think their correctness does not matter much. Sex is important for man.
After a bit googling, I foud :

• for the thrill? Or the sex?
• testosterone, they want more, the thrill, like the power
• lots of reasons

The article under the first link mentioned that biologists think it is for spreading their genes. If you accept Darwinian selection, this is the only logical explanation, all the rest is the consequence of this first one.
There are a few advantages for a man to cheat :

• having more children, so spreading his genetic material
• it costs not much, because these ‘bastard’ children do not live with him (all these unmarried mothers) so he does not have to work for them

Naturally this means that these bastard children have a lower chance of survival to adulthood, because he is not there to take care of them.

Cheating also has risk for a man :

• getting infected with nasty diseases
• clash between the ‘other’ woman and his own wife, ruining his family and lowering the chance of survival to adutlhood of his ‘legal’ children
• getting beaten up/killed by the man of the ‘other’ woman (unless she is single, then it is the father who’s the danger)

It seems that a man is a creature willing to take risks, otherwise they would not cheat that much, but actually it is simply the case that put in terms of darwinian survival of the fittest the advantages overweigh the risks.

So why do not all man cheat ? Some are perhaps more risk-averse?