I know lots of cool guys that would think that would be a hoot. I so wouldn't worry about it. As long as he knows he's the only guy, and is cool with it, good for him! Invite a few more of your coolest guy buddies, what the heck. I seriously can't imagine too many guys who don't/didn't have moms, sisters, wives or daughters at some point. Most of them are pretty comfortable with "girl stuff" by mid-adulthood.