Cheating

I'm so fucking confused. A very good friend of mine told me a few days ago that he and his girlfriend (also a very good friend) broke up after being together for three years, living together and parenting two cats, because he cheated on her with ANOTHER ONE OF MY FRIENDS????? and the story is pretty complicated and he regrets it and everything, but now I'm so distraught by him doing something like that, when I thought I knew him and that he wouldn't be like that. Is he just a man after all?... idk why I'm sharing this I'm just so confused