Why do guys cheat on their pretty girlfriends?

I have a guy friends who like me a lot. He has a girlfriend with long blonde hair skinny and really pretty. I’m not blonde, I have black hair, I’m skinny but I do have some weight. They have a kid together and getting ready to have another. He texts me throughout the day telling me he really likes me and he hates that me and him can’t be together. This is the third guy friend I had who is married and have done this to me.
Why do guys cheat on their pretty girlfriends?
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