Sorry- know it sounds harsh, but my answer would be- dump him pronto. He is an all round bad 'un, having treated not only one, but two, people badly. Obviously, don't know your circs (ie. whether you have commitments with him, such as a joint mortgage or whatever), so don't know how easy it would be for you to extricate yourself from this situation and I appreciate that my advice might be overly- simplistic. But you certainly need to think long and hard about your future with this person, and whether you could trust him again.

i support the first comment completely - and yes it sounds harsh and it's very easy for us to say it and completely different for you to actually do it, but in my opinion if he's willing to share one of the biggest connections and commitments (sex) with someone else whilst he's with you then he's worthless. To me personally that's the worst thing that someone can do to someone they "love" and i personally could never forgive someone for that. At the end of the day though it's your decision and you will have other things to consider in your situation. The problem comes if your trust in him is broken. if it is then your relationship is pretty much dead. No trust makes a relationship near impossible, and it's also near impossible to regain lost trust. It will always be in the back of your mind