Small Penis Problem

Due to a birth problem, I have a small penis, like maybe 3"-3.5" at best with an erection. I love my girl friend very much and she loves me too. Last night she made a strange request and asked if she could try to have sex with a guy that is a little more endowed like 5-6". She promised to tell me about it if it were to happen as she doesn't want to keep anything from me. I told her I would go along with the idea as long as I can help pick the guy she is going to have sex with. This might seem very foolish but I feel my girlfriend is being cheated by my not having a normal penis and, because I love her, I am willing to try it. Is this abnormal behavior? Am I totally nuts?

Thank you for your reply. I appreciate it but just tonight on the phone, my gf said "I have a story to tell you but not tonight." So I made the comment "How about Valentines night"? and she said, "Yes, that would be good." Those are pretty well the exact words. We have a motel room set up for that night. Just from the way she talked over the phone, sort of different you know, I think the deed has been done.