Hey, Aussie. Wow, your girl sounds like mine saying that she is only satisfied with my cock. Bless their hearts, right?
I did mentioned to mine a few times that I wouldn't mine seeing her take a BBC so long as she either takes pics of the two of them going at it or that I am there to watch and/or participate in pounding her like crazy. Hehe
However, I don't think she took me seriously. Now, did you confess this fantasy to your woman or did she really just pull this idea out of nowhere? She may have seen IR pics and vids and is secretly fantasizing about BBC. From what you have said here, everything sounds good! I think you should proceed slowly and play it all by ear. As for asking your woman straight out, ya that's how women are. They won't admit to stuff like this because they are torn between living out a fantasy or keeping it just a fantasy. Also, because they don't want to be seen as cheap sluts. Anyways, don't lose heart. Keep at it. Baby steps. But do take the lead. If you don't watch IR porn, start doing so. And for sure get her the most realistic and fattest BBC dildo her pussy can handle. Good luck, man!