Yes, your fantasy is a little unusual, but mostly harmless. It does seem like extended foreplay and a bit of a teasing seesion. Nothing wrong with that! Afterall, anticipation is one of the best aphrodisiacs. What I think your girlfriend could have a problem with (although I don't know her and she could be totally up for it), would be the desire to get others involved. But the only way to find out is to ask her.