Whether Brad Pitt actually cheated on Angelina Jolie or not is something that only Brad and perhaps a woman or two will ever really know. It was contemplated that he did. He cheated on his wife at the time he met Angelina, Jennifer Aniston, so why would he not cheat on Angelina? Maybe he didn't though.
He was head over heels for Angelina and they had children together which is something he and Jennifer did not have going for them. But, perhaps he got tired of having so many children and Angelina being pregnant. No one will ever know for sure except for the parties involved but everyone certainly has their opinion...yay or nay.