They made a mistake. Often, guys don't really know what they want out of a relationship with a woman, but they don't realize it until they're with the wrong one. This is more common than you think. Many men don't marry for love, but for appearances. They often go with the woman who looks good on paper, but doesn't necessarily lights their fire, so to speak. They find that spark when they're married or engaged to the one who looks good on paper and thus, infidelity ensues.