What's wrong with the official stance of the government with regards to underage sex be "don't do it"?
Because it doesn't work, and has never worked. Hormones are raging and teenagers haven't developed those parts of the brain which deal with common sense and impulse control. Those are the last areas of the brains which develop. That's why teenagers are notorious for doing stupid things.
Giving them the best information possible to protect themselves from foolish decisions is never a bad thing. Especially considering that comprehensive sex education reduces the curiosity factor and actually encourages teenagers to remain celibate. Teenagers who don't have sex education are more likely to engage in early sex.
Because it doesnt work, indeed.
As already noted, Oklahoma school districts have acknowledged the abject failure of their abstinence only programs, and are abandoning them accordingly.