These individuals study End Times prophecy and believe that one of various scenarios might occur in their lifetime. While some Christians (and even people of other religions) believe that the Rapture will follow a period of Tribulation, others believe that the Rapture is imminent and will precede the Tribulation ("Pre-Trib Rapture "). There is a wide range of beliefs and attitudes in this group. They run the gamut from pacifist to armed camp, and from having no food stockpiles (leaving their sustenance up to God's providence) to storing decades' worth of food.
However, a few unintentional similarities to the Quiverfull movement doesn't mean that preppers can't still care about safe sex. Hunting, canning, and digging your own latrines does nothing to make the threat of an STD less real. After all, gonorrhea and genital warts are going to be a whole lot harder to treat without reliable access to medical care. And there must be at least a few survivalists out there rational enough not to want to endure the horrors of premodern pregnancy and birth unless absolutely necessary.