I always heard that it had more to do with the scriptures regarding "woe to the pregnant woman and the nursing mother in those days" than the desire to focus on serving Jehovah. The JWs I've spoken to about it seem truly concerned that, during the tribulation, things will be especially difficult for pregnant and nursing mothers, and they also don't want to subject children to the misery of the "last days."
I know a thirty-something JW couple who, although they've been married for over a decade, have not and do not plan to have kids in this system for those very reasons, even though they really do want kids. I just picture them in their golden years, still in this system, with no adult children to care for them in their advanced age and no little grandchildren to bring them joy. They're probably going to end up being that creepy childless couple that simply ADORES all their friends' and relatives' kids just a liiiiittle too much.
Too bad JWs are discouraged from pursuing higher education, since that often leads to better pay, and with a decent enough income, they could adopt. And just think about how perfect adoption would be for a JW. They wouldn't be bringing any new children into "the last days of this satanic system" but would be helping a child that already exists. They'd finally have a child to love and impose their religion on. I'm actually kind of surprised the Watchtower hasn't come to this conclusion yet. :p
