I'd like to see more detail about the claim that having children is likely to make people less happy-- in particular, whether it makes the vast majority of people less happy, or whether it makes some people unhappy, and leaves others at the same happiness level or perhaps even makes them happier. If it's the latter (and I suspect it is) then rationalists would have a better chance of evaluating whether having children is likely to improve their lives, and more generally, there would be ways to find out how to make it easier for parents to enjoy having children.
There's no question having a child is highly variant in its outcome, both in terms of happiness and otherwise. It makes some people radically happier, and some people radically unhappier. If the effect was reliable then given its magnitude we wouldn't have any difficulty recognizing it casually.
So I found this post quite interesting:
http://www.gnxp.com/blog/2009/03/gnxp-readers-do-not-breed.php
(I'm quite sure that the demographics of this site closely parallel the demographics on Gene Expression).
Research seems to indicate that people are happiest when they're married, but that each child imposes a net decrease in happiness (parents in fact, enjoy a boost in happiness once their children leave the house). It's possible, of course, that adult children may be pleasurable to interact with, but it seems that in many cases, the parents want to interact with the children more than the children want to interact with the parent (although daughters generally seem more interactive with their parents).
So how do you think being child-free relates to rationality/happiness? Of course, Bryan Caplan (who is pro-natalist) cites research (from Judith Rich Harris) saying that parents really have less influence over their children than they think they have (so it's a good idea for parents to spend less effort in trying to "mold" their children, since their efforts will inevitably result in much frustration). And in fact, if parents did this, it's possible that they may beat the average.
(This doesn't convince me in my specific case, however, and I'm still committed to not having children).