When I went to answer, I realized that I haven't really ever been asked that. I guess most people just assume all our kids were "surprises". Truth is, besides our first which was a HUGE surprise, all our other pregnancies have been planned.
What I usually get is "you're done now, right?" Especially now since we're "finally having that boy!"

As for the original question, which I do think is completely rude, I would tend to brush it off with most people-saying something funny like "of course, we decided we wanted a larger tax deduction" or something similar. Even though ours have been planned, and we're not really questioned if they were or weren't, with most people we just play it off as a surprise, because I don't have any interest debating whether or not we should be having any more children with anyone.
ETA: The one(s) that really get me are when people ask things like "Don't you know how this happens?" I usually respond, "No, would you please explain it for me?" Or the one that makes me the most angry-"Don't you know what birth control is?"
