Notice: This is a little wonkier than we usually prefer to get around here, but one of our favorite lurkers asked so we wanted to oblige. Topics like this are generally the province of Lutherans and Procreation. If you're interested in a hard-hitting policy debate on this issue, we're sure the folks over there would gladly set one up for you at your request.
Rebecca asks about infertility (scroll down to read all her comments). To summarize: infertility forces an identity crisis on a married couple. Being neither parents nor celibates, which life should they emulate?
I'm unqualified to give an authoritative answer to, you know, any question whatsoever. So I'll just say this to kick off whatever conversation people may be interested in having: I don't think people who suffer from infertility are under an obligation to come by as many children as they possibly can any more than people with typical fertility have to track their temps to maximize their chances of conceiving. I take the babies God gives me over the course of my normal life and work in other stuff as it fits around them. If God doesn't give you many or any babies to work around, well, I guess that is an opportunity to publicly serve him and his people more than those of us who closely tend a particular group of God's people. (But I don't think that infertility should be taken as a "sign" that a person is meant for some other specific task any more than their eye color or appetite should.)
Does Scripture speak to this directly? I'm not coming up with anything beyond standard Lutheran approaches to sanctification and vocation. Almost every barren woman (and even one virgin!) in the Bible ends up with a baby. Michal remains childless, and it's the last thing we hear about her (she had kind of an interesting career up to that point).
This calls to mind Reb. Mary's adoption post. There are some burdens that most of us with rapidly growing families just aren't in a position to help bear. Childless couples are, generally speaking, more able to serve the Lord by adopting children, and many are interested in doing so. The blessings of parenthood are certainly available to them through adoption, and there is much to be said for seeking out this honor and its benefits even when they are not granted by normal means (to say nothing of the benefits for the children).
But neither is there any shortage of sheaves to be borne away. I think a childless couple or a smaller family is ultimately free to use their time and resources in whatever service to the Lord they feel suited. If they have the desire and finances to adopt two or 18 kids, wonderful. Or they can be like faithful Aaron, holding up the prophet's hands. Their exclusion from one virtuous enterprise leaves them free to choose among others; they may but are not obligated to pursue an office which has not been given to them.
There, I opened the can. Anybody who's interested, get in on some worms. I, a total idiot, am very open to correction.