Answer: For the record, most reasonable people wouldn't take offense to a "Gack, no!"-type answer, since it isn't a commentary on any particular kid, it's just a hyperbolic way of saying you're fine with a life that doesn't involve wiping snot off anyone's face but your own. As for the unreasonable people, you can't hold yourself responsible for those who arrive at a conversation primed for offense.
Nevertheless, as you rightly recognize, people struggling to have children might be reasonably upset by flippancy - and since you won't always know whether someone's in that position, you will need to temper your response to all, not just to those you know are struggling.
In your favor is that your (admirable) awareness will show, so your words don't need to work as hard to be sensitive. You can also shift the topic into safer territory by speaking to the value of choices, versus the value of kids. "No, our life suits us this way, thanks"; or, the goofier the better, "That's not under our tree/in our cards/plans/inbox/tea leaves/fortune cookie/(your quip upgrade here)." "Why?" they ask. "Yikes, that's awfully personal." Self-deprecation works, too, as long as you're willing to risk people who beg to differ when you say, "We're not the nurturing type."
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