Sorry about the delay in responding to this. I kept thinking up long answers which would be best used as blog posts in their own right. But regardless, the “short answer” would be this:
My opinion has not changed on the futility of individual action in order to elicit large changes, in many popular activities (e.g. politics, much vegetarianism, etc.). However, it’s important to understand that there’s a difference between attempting to change the world and what I’m doing (both in not-having-children, and in not-eating-meat alike), although there is some overlap:
In both cases, I’m engaging in behaviour that I hope others would emulate: I’m of the opinion that both of these actions are things that are better for the world and humanity in general, at a value that pays-off over the cost.
However, this in itself is not my primary motivation in either case: I don’t expect nor plan to change the world. In both cases, I’m acting as I am because it wouldn’t be morally right for me to do otherwise. It’s my personal ethics that are my motivation, not a drive to change anything.
To take an alternative example: I try to be a conscientious cyclist – I don’t hog lanes, I let cars out where it’ll be difficult for them to pass me otherwise (I am, after all, a slow vehicle – relatively speaking), and I try to avoid acting in a way that’s inconvenient or dangerous to pedestrians. It’d be easier, sometimes, to be a dick of a cyclist, but it wouldn’t be right. Similarly, I wish that all cyclists were more conscientious: but I don’t expect to change the average by my efforts, not by any meaningful amount. However, I persist in doing what I feel is the right thing simply because it’s right.
I can argue for the (objective, generally) reasons it’s right and I can follow that advice myself, but I can’t force anybody else to do it, although I hope that they will. But the more important part of the process to me is that I don’t feel like I’m part of the problem any more than I have to.
It’s a personal battle inspired by a universal crusade, but the fact that it’s not changing the world does not change the fact that it’s a personal battle.
Voting in elections is an entirely different situation, for me: my vote taken alone makes no significant difference to the result, but I don’t feel that it’s “wrong” to not-vote. Although as you probably know: in my case I tend to spoil a ballot at general elections anyway.