I've had people ask me what my birth name was, again with the 'What's your real name?' thing. It makes me really mad, it's none of their business, and it's private information. I can understand most people don't realise how touchy the topic is when they mention it, and it may just be idle curiosity, so I don't mind just saying, 'My old name doesn't matter, that's not me anymore.'
But if they question further than that, after it's been made clear that I won't discuss it and that it's none of their business, then they're an arse, plain and simple, and I don't need them around.