I find it so funny how so many guys are all for fucking a woman in the ass, but then when the question is turned on them.. "oh no, exit only!" How does it make the man gay but perfectly hetero for the woman... Such a stupid rule. People need to get secure in their sexuality so such trivial things can be looked into.
I'm sorry Ambiguity, but your post sounds pretty condecending.
ktspktsp, sorry but I don't know a thing to help you out. All I can say is that a strap-on will involve a LOT more area for friction so make sure to have LOTS of lube so that it goes in smoothly. Aside from that I know nothing, lemmie know how it goes tho! And Kudos for having a girlfriend who is open to such possibilies.