I know what you mean Jilly!
I used to get really angry, but these days I just laugh it off and say something about how my cats keep me busy enough, or something like that. And change the subject.
I've come to see it in a bit of a different light -- people aren't always being critical of me. Most of the time, in fact, they are just "making conversation." Just like people ask, "So, what do you do?" It is just one of those things that is culturally ingrained to ask when you first mean someone. And you can't really spend time getting all upset every time it happens.
I meet a lot of people in my work, and only ONCE in the past 10 years has someone actually pursued it further. It was a man, and I was pretty surprised. He asked if had kids. I just simply said no, and then he said "Well, do you want any?" Considering I had just met this guy a few minutes before, I thought it was strange. But he wasn't being antagonistic -- more just curious. I just said something like, "Actually, no." And he dropped it.
I come at it with the idea that I may be the very first person they've ever encountered who doesn't want kids, particularly among the older people I meet because of my job. Back in their youth, a woman who didn't have kids was someone everyone felt sorry for because she probably COULDN'T have kids. I like to feel that I am teaching them a little bit about this "new generation" of women who make choices. I am relatively young (33) and successful, and I don't think I fit their pre-conceived notion of what a "childless woman" is. (Which is why we don't call it childless, but child free in the first place!)
I kind of laugh inside a little too, because they are interested in that part of my life, when I'm not interested in it myself!
It's better to greet the world with a little bemusement than to be angry and looking for a fight all the time. That kind of attitude is nothing but draining and a colossal waste of energy. I think it has more to do with whether you look at life as "half full" or "half empty."