If kids are used to seeing violence on TV when they're very young, maybe the Rethugs think they'll be ready to fight in their wars when they reach 18, but they can't drink a beer or smoke a cigarrette until they're 21.
Kind of the same thinking as the anti-abortionists - they need cannon fodder and low-income workers to keep the military-industrial machine running smoothly, but the anti-sex thing is nothing more than moralistic grandstanding and hypocracy.