Who can explain why men have affairs, or women, for that matter?
I’m not sure that it’s all that difficult to understand. From a very basic position, it’s just another sin and fallen degenerate mankind is born in sin....therefore no surprise that unregenerate people do what comes naturally to them in that state, they sin. There is one basic aspect to affairs though that rarely gets talked about.... while affairs sometimes happen when there is an ongoing sexual relationship between the husband and the wife, I would bet that it much more frequently occurs when that relationship has come unraveled. Always being available sexually to your spouse is a commandment from God.... and if one doesn’t carry out that duty, they are disobeying God’s commandments. Since there is no weight given in scripture (that I’m aware of) concerning the gravity of a sin, this means that the one who has withheld sex from his/her spouse is as guilty of sin as the one who had the affair. You asked “who can explain why men have affairs” but you could just as easily have asked “how can any person withhold a sexual response to their husband or wife”?