There are three main reasons which men cite for having an affair. Number one, is sexual novelty, sexual variety or sexual fetish which they cannot disclose to their primary partner. Number two, feeling that their partner had let themselves go and therefore, they're feeling loss of sexual interest. And number three, feeling unappreciated at home or feeling that they're nagged or annoyed by their partner consistently. So those are the three main reasons. Two of which deal primarily with a sexual interest and the third one deals with the emotional reason for the affair.
For instance, many men feel unappreciated or ignored after their wife has a baby and pays all of the attention to the baby and ignores the husband. Or the husband may feel that the wife never has time for sex or interest in sex and instead spends all her time on her girlfriends or on her children. The husband may feel that he has an interest in exploring a particular sexual fetish and that he would insult the wife by bringing it up in a relationship. So he feels more comfortable going to a dominatrix or seeking out an escort than approaching his wife about the particular fetish he'd like to explore.