This is fun! From a book called Favorite Wife, by Susan Ray Schmidt. She was one of 10 wives of a man who had a total of 58 children before she left "the life" (oooh, creepy term!).
So ... you've got God up there creating lots of spirit-children with heavenly Mothers (God is a polygamist!), and it would be a very bad thing if they didn't get to be born into fleshly bodies and raised in Mormon homes. (It would be bad not to be born because .... not sure.) So Mormons have to have a lot of children, but the men also need to have a lot of wives so they can maximize their progeny. This only makes sense, of course, if the additional wives wouldn't have otherwise married Mormon men, had lots of children, and raised their children as Mormons. What this story really sanctions is Mormon men recruiting non-Mormon women to be their wives, so spirit-children don't wind up in households run by Methodists or Jews or what-have-you. Hmm! I've got to wonder-- Do Mormon presidential candidates like Romney (5 children) and Huntsman (7 children) actually believe they're saving the spirit children of God and the heavenly Mother by having lots of children? Maybe!