Morals from an evolutionary stand point. If women gave equal oppertunity to all men, then this would ultimately lead to our evolution coming to a grinding halt and possibly even thrust in reverse. How? Procreating with slutty women would lead to slutty off-spring that would sleep with less genetically gift partners, which would in turn become more sluty and less genetically gifted. The gene pool would become incredibly shallow, and civilization would regress.
I am sure you've read Freakenomics and the authors views on abortion. But if you haven't here's the low down. He writes that at one time in Romania, abortion was outlawed. It caused higher crime, and poverty rates, and the eventual over throw of the government. My point? The apples don't fall far from the tree. So impregnating, either by accident or on purpose, slutty women, is NOT good for mankind.
But given that we live in countries where birth control, condoms and abortions are available, then one can argue that it is infact moral for you to be sleeping with the cheating wife assuming you don't have an std. You know if you're std free or not, and if you aren't fvcking her, she could be out fvcking some other guy that might have an std. This inturn would have an affect on the husbands risk of getting an std.
Statistically speaking most women that cheat only have one or two partners. So why not make sure those few partners are with someone that is not high risk?
Given this, it all really comes down to trust. What do you define as behind someone's back? Personally I think that dating several women is ok. In the end I know I can pick one that I really like. However, sleeping with married women is only short term so the question is, why bother making the investment if it's not going to make long term returns.
I myself have fooled arround with a married woman so don't think I am passing judgement, I am simply posting my thoughts on the subject.