Brilliant thread! Bringing back some great memories. Some favourites:

I remember talking about guys measuring themselves with a few friends, who obviously asked if I ever had. I told her my size but I don't think she believed me until we ended up back at my place a few weeks later: "Good God...you ARE big!"

My first girlfriend: "You've got to be careful with that thing!"

A petite girl who I was dating for a while a few years back: "Holy shit...this is going to be like being a virgin all over again!"