Is it normal that i saw my wife on an old cam?

OK well i have been happily married for 3 years to a great girl. I met her through a mutual friend who she briefly went out with. Last month we move to a new flat and during the move one of her boxes collapsed and i had to tidy it up. Among the stuff was an unmarked dvd. I would have paid any attention but it had obvioulsy been stuffed inside something. When i watched it i found this was some cam film of my wife and our mutual friend riding in his flat a year before we got married and weeks before we were engaged. 20 mins later i was left stunned having watched them hard at it. I didnt tell my wife straight away. A month later she still doesnt know i found it but i find myself watching it at every opportunity and getting really turned on , is it normal ???