See the post-nuptial shutoff thread. For some women, once the thrill of the engagement, immaculate wedding, honeymoon, new home, new cars, and baby arrive...many wonder "is this all that life has to offer me?"
The woman's libido goes into the garbage, they gain weight, get depressed. Sex life dies.
Meanwhile, the husband is the still as horny as he was at age 15, and because the wife no longer wants to sleep with him, he goes out and tries to act like he's in college again.
I guess the biggest question I have is how these guys have the time to pull off an affair? Seriously, a wife, full-time job, kids...what time does he have to game chicks?