I have known this guy for 18 years and he was my boyfriend for about 12 years of that time. We had a son together but broke up about 6 years ago. I recently found out that he has been sleeping with his best friend's girlfriend for over a year. He has been friends with his best friend longer than I can remember. Why would he throw away this friendship for someone like that? I'm finding it hard to reconcile this person who I thought I knew? I broke it off with him because he was a constant cheater, but... I never thought he would do this to his friend. Why? It makes me question his morals even more as a role model to our young son.

You have found out something you need to know about him, and should be very glad he is not with you now. Having a child together, is a whole different subject, as he is the father and you did chose him. Do the best you can living with your choices.

because guys can be jak asses who only think with there smaller head.
I'm so so sorry for all of that
=[ guys are evil

He sounds like he is a very selfish person. Someone who cheats all the time then just starts sleeping with the girlfriend of a close friend. Oh jeez... some people really only care about themselves and do not care about how their actions may injure other people. I imagine you figured this out as he is no longer your boyfriend.

The best thing you can do is have other positive, healthy male influences in your child's life so that they do not believe that this man's actions are the actions of a "real man".

Because many men were given by a cruel twist of fate two heads but only enough blood to run one at a time.

The real reason people not just men are stupid and do stupid things. They lack the ability to think beyond the immediate gratification of their actions.