Why do men cheat?

Hi everyone, this is my first post on reddit. I will withhold any information about myself and others as I respect privacy. For all my relationships I would be a second choice or what they call a ‘backup’, I assume I was taken advantage of. I am very trusting towards people and I get very attached to them, that's why I go through breakups very hard. My last boyfriend (let's call him Joe) seemed to be sincere towards me, but how wrong I was when we were separated by distance due to studies. I was so hurt when I found out that Joe had planned to break up with me a long time ago (I learnt this from mutual friends). While I was worried about our relationship, because Joe was actively pursuing me during this period, he was with someone else. Although just a week ago I slept with him (we had been together for 5 months at that time). I didn't tell anyone about our break up as I thought we could still be together. But when I found out how he postpupil with me, hiding the cheating and leading a double life, I hated him. And now I think, if all men cheat, could I forgive his cheating?