There is a school of thought that says if a woman cheats, it is more likely to signal an end to her primary relationship than if a man cheats (link is external). And this may in fact be the case, because men and women tend to think and feel differently about sex and relationships.
Men are generally more likely than women to be able to compartmentalize sex and intimate connections. For many men, sex is sex, and relationships are relationships, and the two do not necessarily overlap. Thus, a man who casually cheats may do so without feeling a significant degree of emotional connection to a mistress, while a woman who cheats could see things differently, with sex and emotional connection intermingled in ways that make compartmentalization more difficult.