Thank you for your replies. I am not so bothered about the cheating, I don't really view it as cheating. If it were a random woman I would have gone mental! I have googled and what this is called is genetical sexual attraction, something that happens when adopted children meet their biological parent for the first time when they are adults. The desexualization phase children go through with their family has not taken place becaue the biological parent was absent. So I am not concerned that he would have sex with any of his adopted family or my family although that theoretically would be a lot less wrong. It is only the mother.

I know he feels bad about it but he can't stop and thus just blocks out the thoughts. He says that if he were to stop and really contemplate what he is doing he would want to kill himself... He is also aware that he needs therapy but is too scared/embarassed to tell anyone about this.

You are right, the mother was indeed the instigator and I am starting to resent her very much for allowing this to happen. Although both adults, the parent is always in a position of power with their child. Prior to this starting he had been desiring her for 6 months and felt absolutely horrible about it and very confused but he didn't do anything about it. Of course that is the time when he should have gone to see a psychologist and this would never have happened.

When he met me he stopped having sex with her for months so it appears he can stop. But I asked him why he started again and he said because she kept wanting to. I assume because that is the only hold she has over him and she is terrified that he will shut her out of his life. She often acts like a needy girlfriend.

I know the sensible thing is to run fast and far...
But it isn't that easy when you love someone. We are in a serious committed relationship, and we have a great time together. He has had two long relationships before me but they didn't know his dark secret. He has been living a double life for so long.

I wish I didn't know. I hate that he told me. At the same time he showed me more trust than he has shown anyone else ever by telling me and it feels very wrong to leave him because of that. He tells me the truth about everything as he feels he just has to, that there can be no lies or secrets with me. I see so much potential between us.

Our relationship has from the beginning felt fated, you know like written in the stars. I can't shake the feeling that maybe I am supposed to help him work this out. I am just not sure how to handle it, what to say to instill in him the will to stop without putting him on the defensive or damaging him and our relationship?