It is because your trust was betrayed. I my case, it was by a Scoutmaster when I was 10. I was brought up to trust authority figures. When I was SA by someone in authority, all that trust disappeared. I could never trust anyone after that, and I really did not realize just how deeply that went until recently.
My wife had the same issues at first. Here we had been married for 23 years, and I never confided in her about what was going on inside my head. She knew something was wrong and that I was not being faithful. She was furious that I did not trust her enough to come to her and tell her everything so that I could get help. When she threatened to end our marriage, I told her that the SA was a lot longer and more of it than I had told her about in the past few years.
She literally gave me my life back when she arranged for us to have marriage counseling with a therapist that specialized in SA. She also said that I have to put complete trust in her.
Me placing that trust in her has been extremely hard on both of us, but has been the greatest thing I could have ever done.
As survivors, we learn that all things are based on lies and that we can trust no one. IMHO, the first step to recovery is going out on that limb and having someone to place complete trust in. It is easy with a therapist, you can open up and let it out without fear. But to do it with your partner is a huge step!
I would talk with your wife and make sure that she understands that she is going to have to be stronger than at any other time in her life in order to help you, and there are going to be times when she is going to really grieve and hurt.
It has been difficult for my wife, but it has been such a blessing at the same time. I can lean on her for stregth when I need it, and our marriage is the happiest it has ever been.
REMEMBER, in my case, (which is all I can reference to) I put trust in my wife, but all the lies and secrets came out just a little at a time and she usually forced them out of me. It is a very painful experience for both of us when that happens. But in my case, in has been necesary for my recovery and healing.
Didn't mean to ramble on so long, but withour my darling wife, I couldn't have made it this far. She is committed to my recovery and the betterment of our marriage, so the other point is she HAS to TRUST in my committment to staying with her at the same time I TRUST her committment to me.
Hope this helps!
Mike