I wouldn't do it.
Your putting your relationship with your girlfriend at major risk. Sleeping with her mother will certainly destroy everything you've built with her. Plus you will destroy her mothers relationship with her boyfriend too. Not to mention that both mother and daughter will probably not be able to look the other in the eye ever again.
Its really not worth the fantasy.
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Dirty Harry (13-06-10)
RicFlair (13-06-10)
It could also be a trap maybe she wants you to make a move could all go back to the gf
I have lived a life of regrets.
im sure you have thought of asking them to share you?