Agree with the rest, so not much further to add, except for:
What seems to particularly rub people the wrong way is my suggestion that this is morally obligatory. While my views have not shifted greatly I've learned enough from this trainwreck of a post to argue this position less stridently next time around.
Yes. The mostly-utilitarian environment around LW already doesn't support moral obligations, but on top of that due to the various issues surrounding moral systems it's frowned upon, partially due to the large risk of inducing conflict and confusion, to directly assert a claim like this that results from an assumed moral system.
Even though it seems like the majority of LW would "support" it, a post made entirely about encouraging people and justifying a case for the point that it should be morally obligatory for everyone to make expected utility calculations in a trolley problem and push down the fat man would not be that well received, I think.
An approach that, I think, would be much easier on this same subject with intellectual communities, particularly LessWrong, would be to claim that your point of argument (People X "should" have children!) contributes more towards some goal (Higher ratio of quality humans?) than alternatives, and is thus closer to optimal in that regards (if you claim something as truly optimal without any caveats and an extremely high probability, you damn well have the durasteel-solid math to prove it, or you deserve every criticism and tomato thrown your way! not that I'm guiltless of this myself).
EDIT: And to complete the last thougth above, which I thought I had written: And in most intellectual communities, the gap between "closer to optimal" and "moral obligation" is then easier to cross if one really wants to insist on this point. Arguments could be made that any sub-optimal is harm by opportunity costs, or about the relations of individuals' utility functions to social factors and thus to their behavior towards these "moral obligations", or various other ethics thinghies. Basically, it's just a more stable platform and a better meeting point for launching into a pitch on this subject.
This topic is in vogue, so here's my pitch.
My fellow humans, I have some bad news and some good news. The bad news is that you are likely to eventually enter an enfeebled state, during which you will not be able to independently provide for yourself. Even worse, you will at some point altogether cease to function and then you can no longer contribute to the things you care about. The good news is that both of those problems can be ameliorated by the same scheme – the creation of new humans. The new humans can provide us with the assistance we need as our own abilities diminish. And when we cease to function, the new humans can carry on with the projects we value.
Now, the thing is, creating fully functioning new humans is a huge project, consuming many man-years of work. A person engaged in preparing and outfitting a new human will need to sacrifice a lot of time that could otherwise be devoted to personal leisure and other projects. We currently have a volunteer system for replenishing the population and in many ways this works well. Not everyone is well-placed for creating humans while some people are in a good position to create many. But this system is not perfect and it can be exploited. There are some freeloaders who do not create humans even though they are in a suitable position to do so. Those same people almost always value receiving care in old age and value humanity having a future. But they are relying on the rest of us to provide enough new humans for this to happen while they can devote all their time to other projects and zero time to diapers with poop in them.
Sometimes the non-child-creators justify their decision by suggesting that the projects they are working on are especially socially valuable and thus they can spend time on them in preference to child-creation without violating their duty to society. While it is *possible* that this argument goes through in some cases, it seems suspiciously self-serving. What is especially worth taking into account is that if the humans in question really are so highly valuable, they would statistically have highly valuable offspring. Thus, it seems doubtful in the general case that high-value people refraining from procreating is a net gain for society.
[Poorly conceived section on my personal experiences removed.]