He cheated???

My boyfriend and I have a difficult situation. From the begining, I knew he was still going through divorce when I met him. Besides my better judgement we continued to date and we now live together. After being in a relationship for 18 months, and living together for 7 months, I found out he Slept with his "ex-wife" (longest divorce of my life!!!). I was devastated... I honestly still am and I don't know what to do. He says that it happened when we had, had a fight, and he though our relationship was over. But with his ex? I know some couples can make it past this, and I know some don't. But I have no idea what to do! I love him, but why her? To make things worse they have children together so I know it's not someone I can just cut out of his life and hope he never talks to again. I know that he had to maintain some type of relationship with her. To top all of this off we were trying to have a baby... I have not been able to get pregnant and ever since finding out I have put that off. Please help!