Marriage, infidelity & ?

Memorial weekend my husband told me he had been having an affair that ended badly. He realized that I am his soul mate & we're working on strengthening our marriage. I want to work this out as I believe he is a good person, he just made a bad mistake. Before I found out about the affair, I was aware that he frequently visited web sites. I don't have a problem with that either. However, my 11 year old granddaughter also uses our home computer and I wanted to monitor what she was doing so I frequently checked the history on the computer. She is 1/2 African American & 1/2 Caucasian. I found a site that was entitled "Black teen girls" which was a site. I've asked my husband about it last night, knowing that he had this affair & knowing that he visits sites. I told him that I'd kill him if he ever touched her. He blew up at me and is now barely speaking to me. When I asked him what's on his mind he said "you do not want to know." He pretty much has clammed up. He is not related biologically to my granddaughter. She is definitely starting to develop. Should I have not addressed this with him? Before all this, I did inquire with my granddaughter about Google searches & black teens. I did not tell her what the search was for. She denied it saying it would be racist to put "black" girls when searching. Any comments, suggestions would be appreciated. I already feel horrible about the affair - I want to believe my husband - what would you do?