Questions about sex life

Why do people who are not my best friends think it's okay to just ask me if I'm having sex? In my opinion, that's none of their business. If I wanted them to know, I'd bring it up myself. I'm not having sex but when I say that I don't want to answer that question, they just assume that I am. Some people just don't feel comfortable telling people about those things. I've had people tell me that I should be having sex so I don't like telling people that I'm not.