Who's saying that to you?! I kill 'em!
There's a few people in our hall who've asked if we're gonna have kids, and my usual response is that if they knew me well enough to ask me that question, they'd already know the answer; and if they don't know the answer, then they don't know me well enough to ask. I've only had to say that to a few people because of the degree of snottiness. It works.
but then, how will they GET to know you if they don't ask?