Love your blog title - I love all things sparkly!
It is very, very common to have a gut reaction of 'I can sleep with her by myself; we can sleep with her; but you are sure as shooting not sleeping with her by yourself.'
Now if he is ok with that (after you admit it sheepishly) and she is ok with that, well, people had made weirder arrangements work. It is truly up to the folks in the (at this point potential) relationship.
And it is good that you recognize you have this reaction. Not everyone is that self-aware. So take a look at it. See what is 'really' fueling the reaction. Are you afraid of comparisions? Fear of losing your special place in his life and affections? Fear of being replaced? (I'm not saying any of these is the reason for your reaction but they are some of the more common underlying emotions.) Reactions like these are sometimes easier to cope with if you understand their root cause. (And sometimes knowing the root cause doesn't help end the reaction but that's another post.)
*Adds 'pork-n-beans' to my fetish list*