He's lying. Cause he's embarrassed
You shouldn't have made a big deal out of it and not call him

If he was holding it for a friend you would have found it on the coffee table, not under the couch. I myself have been cheated on in the past, and I know how you feel. Personally, I feel Porn is cheating but not everyone feels that way. The internet thing would disturb me and I would talk with him about it. You deserve to know the truth. If he has never lied before, than you have a good foundation to start from.