They’d better permit polygamy for everyone or they will find the breeding rates among muslims with multiple wives out pacing all other groups.
Not really, the Catholic Mexicans, and the Mormons, even the non-polygamist ones, will give them a run for their money.
What counts is the number of babies per woman. With more than one woman to for the men to stand stud for, overall rates might actually be a bit lower than they otherwise would be.
The bad news is that all the surplus males make great holy warriors. And they are by definition, expendable.