When someone comes up to you and asks if you're gay/lesbian what do you do? Do you run the other way without saying anything? Do you say that you aren't? Or do you tell them that you are?
My suggestion is to do what feels the most natural to you. But don't just run away because someone asked. If you really don't want to tell them that your aren't, then just tell them that you don't want to talk about it. And if they keep asking, just walk away from them. I can understand if you say that you aren't simply because you aren't ready to be out. And sometimes you are able to tell them. Any reaction is reasonable in my mind.
Sometimes people will ask you and you aren't ready to let people know. And that's fine. But remember that many times people ask you if you are gay/lesbian because they care, and just want to know so that they can support you. Other times when someone asks you and you tell them that you aren't gay/lesbian because you aren't ready to be openly gay. And I've done that. I've done that with people up until the last year and a half or two years. And I told people that I wasn't ready to answer that question. And people have been so understanding when I've told them.
But there are times when you are able to tell someone that you are because you're ready. And I've had that happen too. One day I was standing in a line at Walgreens, and a guy behind me asked me to go out with him. And so I told him that I didn't, and that I was gay. I think it took him by surprise that I was so open about it.
So when someone asks, don't be afraid.