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).
I realised early on I wanted children, and now I have one. (Watching this small intelligence develop is utterly riveting.) I'd like another, my girlfriend already has two others and really doesn't want a fourth!
This is not something that came from a rational decision. The desire came first, so the job of instrumental rationality as far as I was concerned was to make this work.
For those who have some desire for children but can't rationally make that work: may I strongly suggest sperm or egg donation. In the UK, they're terribly short of donor gametes (a shortage of over 200 donors a year), and having smart, rational people donate would probably be a net win for the gene pool at little personal cost (more faff in the case of eggs). If your sperm or eggs cope with freezing, you will reproduce.
If you're worried about your offspring being cast to the four winds, I wouldn't be too concerned - I was adopted, and it worked out I think because I was quite definitely wanted, not an accident. The children of your sperm or eggs will be very much wanted. It gives women the hitherto-unavailable option of passing on their genes with someone else doing the work of pregnancy.
And if the singularity takes more than twenty years, you'll have added a human of higher intelligence to the population.
In the US I've found most clinics are actually fairly picky about the traits they will accept in egg "donors"--who are usually reasonably well-compensated, which is, I suppose, why they can be so selective.
(I think it might be nice to pass on my genetic material. But no one paying through the nose--or any other orifice--for genetic material wants mine. Can't say I blame them.)