This was really a common discussion in college, but I'm sure it takes place much sooner nowdays? It isn't meant to be intrusive, as it really is, but I guess just another one of those girl topics to help feel "normal."
I might suggest if they (other than a doctor) asks when... ask them what makes them think you ever did? (lose your virginity?) Maybe a smirk or snear will back them off and still give them something to talk about.

There was a discussion in a thread here once that approached the same topic. Only I think it ran along the lines of whether you were still a virgin if you never engaged willingly? It was a good thread, imo. You know? What defines "virgin" these days? I know a more recent thread that talked about not being sexually active if you only did other stuff and not the main activity. That was quite a thread!
Try and remember that most of the people asking these questions are not being very serious about the whole topic. Doctors, however, well, I'm glad you were able to disclose.