I've even tried the "answer their question with a question" routine. Like, "why do you need to know?" or "why do you ask?". If you can survive the awkward silence that follows that will usually solve the whole problem. I don't like situations like that so I don't use that tactic too often.

But, like Sandy said, those that I really care about know me and my situation and don't hold it against me if I choose not to go out. Those who do......well, I haven't lost anything.