lol, this is one of those things that is so explosive i don't want to actually give you any advice.
you could go through with it, get jealousy issues from either you or your lady friend, and have it ruin the relationship. or, you could have a really good time and be having more girl-girl-you
threesomes than i ever dreamed of.
i mean, i wouldn't want to see my girl with another guy, but i have no idea how you feel about that, so i really don't know what to tell you. i would also rarely bump into this situation because i tend to attract either socially conservative girls or girls who pretend to be socially conservative, but are really closet sluts.