It isn't just religious fundamentalists - it's also people with low conscientiousness. They have no moral aversion to birth control, they just don't think too hard about the consequences of unprotected sex (it feels nicer, and condoms are at the drugstore three blocks away).
And while Mormon's rapid breeding might prevent demographic malaise, the rapid breeding of low conscientiousness people won't - the very trait that causes them to breed rapidly also causes them to be generally unproductive.
And while Mormon's rapid breeding might prevent demographic malaise, the rapid breeding of low conscientiousness people won't - the very trait that causes them to breed rapidly also causes them to be generally unproductive.