I just say no. I think most people realise I have no chance of having a gf when they see me, so most won't bother asking follow up questions. Funny sidenote: Back when i had just started university (at age 18 or sth), I was sitting together with 2 dudes in a lecture and one of them asked out of the blue if I was a virgin. I said no at the time, but could see him not actually believing it (why would he even ask that question if he doesn't already think that i am). I don't think i have been asked the virgin question since then, don't know how i would answer now