Wow, big subject! Lots of comments. 

I think a reason it's so stressed is because sex effects us so deeply. No pun, but it gets under our skin, more powerfully than so many other things (1 Cor. 6:18). I want our children to be equipped with some appropriate knowledge so they can make decisions for the good of their futures (and out of compassion, their girlfriends' futures). The reason the message is so mandatory is because the opposing message is so loud. Holy smokes, it's loud. There is sex everywhere -- as a kid/adolescent, you'll drown under it, if you don't have someone kind and wise to help you make sense of things. 

I met my husband when I was 16. Sex was a complicated issue, and even as a Christian I didn't know how to navigate it. We didn't have sex until we married (I was 22). This is personal, and not meant to be reflective of anyone else, but knowing what I know now, I feel certain that having sex before our marriage would have destroyed the relationship. I thank God we waited -- that's His grace, not my boast. As for now, I'm secure in the marriage. Sex was a tough issue for me, but that marital commitment was the stronger force. Again, I have to praise God for that; I would have crumpled without my husband's promise.

Compassion is good. "Guard your heart" is also good (after all, a broken, hurting heart can't function as healthily giving). As for marrying young -- darn it, I wish people would stop writing other people's love stories. God transcends our imaginations. If I meet Husband at 21, pray for us, drill us, counsel us, celebrate us, bless us. If I meet him at 62, ditto. And be there during the marriage, not just speculating before it.

(My exasperation is not directed at you, Kristin! -- Love the provocative post!)