Has anyone ever dated someone who went to prison? If you stuck with him faithfully how did it work out?

Wondering cause I met someone we dated a couple of years then he went to prison, he was there about 4 years and i remained faithful to him, although he was faithful to me before leaving, when he got home he cheated on me! and we are still together, but of course i don't trust him any more. I do seriously believe that he loves me though. Why do men cheat, when they already love someone else? And please, don't call me stupid and all that- this is my situation and I would just like some feedback