I did turn a bit detective on my ex, who was similar but not as bad as the OP's. He kept dumping me but just said it was because he was "bad at relationships" (at 40!).
After some FB sleuthing, I found some fkirti g online with another woman around the time he ended it with me. We had mutual friends in common, so I asked some subtle questions and it turned out he had been cheating on me with her. She had no idea. By the time I found this out, he'd already dumped her once and taken her back, only to dump her again.
I added her on FB and we've now become friends. Turned out he used exactly the same phrases when dumping her as me.
One of my other friends I hadn't seen for a while that I also got in touch with also told she'd seen him all over another woman at the time he was in a relationship with me! A male friend then told me that he had actually had a long term girlfriend in a LDR that my ex had sworn him not to tell me about several years before when we had initially got together.
I'm sure there's more and that I haven't found out everything but what I did find out was enough to put me right off him, as he was actually a completely different person than the one I fell for.
My ex presents as a slightly shy guy and insists he likes his own company, needs a lot of time to himself but in reality he just sleeps around and goes from one woman to another. He also likes to cultivate the notion, never quite answered, that he might be gay. Which is actually a really good front for sleeping around and messing women about. It's a good way of getting sex without commitment, which is normally quite difficult for men after the age of about 35, unless they live in nightclubs or are a ski instructor!