Girlfriend Hates Granny Porn

[Spanker]

I have a fairly embarrassing problem concerning my girlfriend. She recently found a copy of a mag containing some very icky granny porn under my sofa. I have no idea how it got there, but suspect it must have been one of my mates left there as a practical joke by one of her mates. Now she can't get it out of her head that I get off to freaky granny porn and thinks I'm some kinda pervert. She won't believe me when I say it isn't mine. This is really awkward. Any advice?