I agree with many others - and again with the dramatic generalizations, men often seem to cheat for more "physical" reasons, while women seem to cheat for "emotional" reasons.
Neither my husband or I have ever cheated, but I also agree with dianed - many men may forgive their wives, but wives tend not to forgive their husbands! My husband is a guy very much 'in control' of his emotions and his body - therefore if he were ever to cheat, I would feel like it was really intentional - not a 'heat of the moment' kind of thing, and I don't know that I could forgive him. On the other hand, I am pretty rational (but more emotional than he is), and I don't have nearly as much willpower as he does. I have never cheated and am completely committed to NEVER cheating, but if I were to, I think he might be able to forgive me and move past it.
mariannes, I don't understand how some think that it's not "all that wrong" to cheat either - I don't think I could get past that!