May I say something even if it will probably make some people upset?
I didn't read through all the responses here, but I'm going to assume nobody has posted anything similar to what I'm about to say. I just wanted to put this out there to give a little perspective because this is a touchy subject for me.
I do agree it's wrong for other people to comment on another person's parenting choices (like when they're going to have another child). But I have to say that as an only child myself, I would ask the parents of an only child when they're going to have another child in order to help out another only child. Some only children may love being an only child and it may work out for them in life. But I think it friggin sucks! For the rest of my life I will always be upset with my parents for not giving me a sibling. You have no idea how horrible it is (for me) being an only child. I think it's actually way worse now that I'm an adult. The older I get, the more upset I get about it.
So I would totally comment about how an only child needs a sibling. I don't care if it's considered poor etiquette because to me, being an only child is equivalent to child abuse. In fact, I would have preferred my parents to have beat me (or worse) than to be an only child. Yes, I actually felt that way as a teenager and still feel that way. I'm sorry if this upsets some people, but unless you're an only child, you don't have the right to be upset because there is no way you can possibly understand.
And again, I just had to say that to give a little perspective. Sometimes people wonder when you're going to have another child because they're concerned about a child being an only child. This wasn't meant to start a heated debate, I just wanted to give my view (a different perspective) on the subject because this entire thread is basically about being an only child and I am an only child. I don't think there's many of us out there. I think there was only two or three of us in my entire graduating class - 160 or so.