Why is it that always when a question is preceded by “If you don’t mind my asking…”, it’s, well, something that I would usually mind someone asking. And people know that, because they’re including the disclaimer.
How do you get out of that one without sounding snarky?

I answer with a raised eyebrow and “why do you ask?” Unless they are totally uncouth, they should at least have the grace to be somewhat ashamed.
One never knows…