In my experience, men in their 20's think that they want to settle down, and that they can be monogamous. They have babies with their Partner/Wife, and life gets awfully serious (mortgage/broken sleep/wife not looking very glam/less money etc).
Time moves on, they hit their mid 30's, and then with great revelation they realise that they have missed out of shelf loads of alternative vagina's. And they are almost 40! Time waits for no man.
Wife (and life) at home seems dull, compared to the Single women at the Office. Why shouldn't he dabble? After all, wifey at home doesn't give him much sex right now.
And there you have it. Marriage over.
Unfortunately, almost every relationship I've seen, has followed this predictable pattern.
Man moves on, and then generally is faithful to Wife number 2, as he realises that he lost everything through cheating the first time. And, after 5 years of on-line dating, he realises that to have one good woman in the hand is worth two in the bush.
Gosh, that sounds awful.
Disclaimer: On my 2nd marriage and we are very serious about monogamy. BUT...there is very little stress, children are adults, we are okay for money and have nice holidays etc, plenty of sex/affection, so, I guess it's easier to stay faithful when things are good.