Yeah, I think no matter what, sometimes nosy/thoughtless people are going to find something to comment on. One kid? You need more. Having a fourth kid? Way too many. Two boys? You need a girl. Two girls? You need a boy. Aren't you a little young, aren't you a little old, aren't they too far apart, aren't they too close together...
Sometimes I don't think these comments come from any deep philosophy, they're just inane chatter that people have heard and are repeating, without thinking about the implications of what they're saying. A polite but cool brush-off is probably the best.
On the other hand, if someone is talking about their own
choices, or seems really interested in why you chose to do this or that, you could hear them out and discuss if you felt comfortable with the topic. Or if not, you could say something like, "Oh, that's nice. Wow, the weather's been nice, huh?"
Some people feel more comfortable talking about intimate topics--my friend Amy, for example, knows all about the reproductive choices, medical problems, monetary issues, etc. with all members of her extended family, and has no problem sharing them with me in a matter-of-fact way. Then I'll tell her something that happened with one of my cousins, and she'll ask me all kinds of probing questions that I have no answer to, because we're much more private. If she ever came to a gathering of my extended family, she would probably come off as alternately nosy and oversharing in comparison, but that's just what's normal to her.