Libby Anne, of the anti-quiverfull blog Love, Joy, Feminism, discusses the natalist justifications of conservative Christians.
Interestingly, the author starts by suggesting that having children will indeed diminish your happiness. But she asserts that you’re supposed to have children because God said so, not necessarily because you want them or because they will make you happy. She does say that children will bring you “joy,” but in evangelical circles “joy” often serves as a code word–it’s the thing Christians are always supposed to feel even if they aren’t happy. The basic idea here is very much a theme in evangelical Christianity—God does not call you to be happy, he calls you to obey him. Your happiness matters less than whether you follow what God lays out in his word (as interpreted by various church leaders, of course).