Maybe because you are "shy"?
Or maybe because you are being shy?
But then again, does "shy" always mean "hasn't been with anyone"?
People make assumptions. I make a whole load of massive guesses on this forum. I get it right a lot because I've been through similar things but boy do I miss the mark sometimes too.
People pick up on behaviours and then make assumptions. They guess and get things wrong. And that's only if they are bothering to be that judgemental in the first place.
It is possible that you feel so ashamed of your virginity (for some reason) that this is a really big issue for you and you have started to think and feel that the way people interact with you means they know you're secret.
You're a virgin. So what?
It's not a mark of shame. Some people just assume it is.
I lost mine at 17 to a 24 year old. I had to put up with the usual crap at school about how late I was in losing it and how sad I was and how they felt they had to up their game because "oh my god, Martin's getting some". Yeah, hearing that made me feel fantastic about myself I can tell you.
The losing of the virginity is not the making of the person.
If people's assumptions are spot on, so what? Do you need their validation?