Let's say that McCain didn't save it for his first marriage. Doesn't that strengthen his case for abstinence-only education? "If only someone had taught him better," and so on? And given that his first marriage broke up, couldn't this all end in a really awful appeal to the protective effect virginity offers to marriages? Now, I think McCain knows exactly how disingenuous and stupid this is, but given that he's saying kids should be taught to preserve hymens, that he didn't get such lessons and so didn't follow them isn't exactly a lethal rejoinder.
