Why do I fantasize about my girlfriend's friend? Can you explain this fantasy?

Had a fantasy about my sexy girlfriend and her hot friend in a 3-some?

I erm, get slightly excited seeing the friend's pictures on FB. Also I imagine them at first both working together to please me giving head.. and then them both trying to outdo each other to impress me?

What does it mean?
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The thought of them both giving me a double blowjob together is really hot? And erm.. giving them both a facial together?

I umm.. fapped about both girls the other day?

What do you think?
Why do I fantasize about my girlfriend's friend? Can you explain this fantasy?
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