How are Mormons supposed to procreate with special garments on at all times?

The same way people who wear underwear all the time do, they take them off. Yes, Mormons do it too.
Although some have been known to attempt to do so while wearing garments, I believe it is not the mainstream practice.
There are times when the more 'proper' thing to do is to remove the garment temporarily, such as in showering, swimming and competing athletically in public, etc.