Hi Juicy B,
I agree with you that saying 'no' to the suggestion since it involved a friend was, in my opinion, the right thing to do. Having done a two male threesome with a friend it went well, the threesome part, but after the threesome it became quite awkward afterwards. We have also had a couple, that we were good friends with, approach us about wife swapping. In that situation my wife was not attracted, at all, to the guy and our friendship with them drifted. The point I am trying to make is that when you include a friend into a threesome there is another element, the friendship, that needs to be considered and how it will be maintained once it ends.
Moving on to the other part of you question regarding double penetration and if it is common fantasy for women. I know it is a fantasy of my wife but I do not believe it would ever come to fruition.
My advice would be to take it slow and to decide, what if anything, in a threesome you are comfortable with doing. Also take some time to talk to your girlfriend more about her idea of a threesome and hopefully the two of you can work at finding a solution that works for the both of you.
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