Joanna, sad to hear about your loss.
To answer your question: Men cheat (and so do women!) because our forebrains are, to put it simply, incompatible with our hindbrains.
Our hindbrains and in fact our bodies are designed for cheating. Just as an example, semen is composed of an interesting broth of substances: sperm cells, nutrients for said sperm cells, and interesting little "anti-sperm" sperm cells, that decimate another man's sperm, if they encounter it.
Fascinating! And there's lots more things like that about the male and female bodies that point to a heriditary "cheating" nature.
However, the main reason why men cheat is because it's written into the very deepest levels of our bodies to spread our genes far and wide. It's simply human nature.
I know that these things I'm saying probably won't go down too well...pun absolutely intended...but that's just my take on the whole thing.
Personally I wouldn't cheat on someone, purely because if I was going to get all monagamous, I'd make sure it was with someone so amazing that they'd completely fulfill me!