Infidelity has always been hurtful for both sexes for different reasons. For the woman it is emotional hurt and betrayal. For men as bad as it sounds, it is a more proprietary thing and, of course it is not the same for every person of either sex. There are and have always been men who believe in complete fidelity but they were rare in my day.
Based on my conversations with younger people, I think the value of fidelity is better understood today and overall men are more sensitive. In my generation it was more or less like the military rule, "don't ask, don't tell".
Lindy's comments on romantic sex are a perfect illustration of the difference in men and women. Women are more emotional about sex and this is the reason that they are less likely to cheat. When you exclude the obvious advantage of a partner that knows you and what you like, most men will tell you that the 15-20 minutes of the sexual act is purely physical. The 15-20 minutes before and after can be quite different. Men are more inclined to separate the physical and the emotional, while women tend to put it all together.
Anyway you look at it, we are guided by mores and folkways that are much more flexible than evolutionary traits