I worked at a bookstore at the time and the magazines were filled with the story. They had just finished renovating a house, publicly talked about starting a family and there were pics of him and Jen on vacation just days before they split. Brad said there were issues, I don't think Jen thought there were. No one but they know. He's a serial cheater though so I lean towards he did.
I remember those stories too. The vacation photo's were strange, I remember in many she appeared to be crying and weepy. Many said that was when he told her she was leaving her.