How about this idea. At thirteen, all kids are given a test of general understanding about sex, STDs, and how babies are made. Nothing complex, overly graphic, or beyond their comprehension - just knowledge any thirteen year old should have in order to keep themselves and others safe. Then, if the kid doesn't get a passing grade, that kid's going to get sex-ed, no matter what the parents want.
Parents say they want to be the ones who teach their children about sex - but then don't do it. C'mon, people, either teach your kids about sex, (assuming you have the resources to do so,) or allow the Education System in your state to educate them.