@hingehead,
It was male fertility which was alleged to have declined. I didn't see anything about an environmental cause, though. Impoverished agrarian societies need lots of children, because they provide labor, and they are the retirement plan for poor farmers--if your son survives to adulthood, you rely upon him to support you in your old age. Industrrial societies have made such transactions meaningless. The family doesn't need child labor, nor are the parents dependent on the children for their support in old age.
I find the entire scenario interesting, but i don't know that anyone has positively shown why the decreasing fertility rates occur. I think, though, that the religious aspect is meaningless. The Spanish, after all, are just a Catholic as the Italians.