Nobody gets left behind
Teenagers are always going to have sex. By putting sex education on a curriculum, no child gets left behind and is left vulnerable, without the knowledge to make informed decisions and protect themselves.
An overwhelming majority of parents prefer teachers to educate their children on matters of sex and relationships.
A 1999 survey among American parents found that 93% of parents wanted children to receive some form of sex education by the time they finish high school. 84% wanted their children to receive the classes before they finished junior high. In a separate survey carried out by The Advocates for Youth, 83% of parents said that they thought their child should be taught how to put a condom on in school. 71% also believed that schools should inform students on where they can get oral contraceptive pills. 86% supported informing children on where and how they can be tested for certain types of sexually transmitted diseases. This indicates that parents would rather teachers taught sex education, or at least offered some information to supplement their own sex education teachings. 
Not all parents want it. And for those that don’t, sex education in schools can undermine their religious beliefs. Jodi Hoffman, a Florida mother, sued the county public schools board for teaching her children about contraceptives, abortion and homosexuality in school. She argued that in educating her children on these matters, they had clashed with her religious values that disagree with sex outside of marriage, abortions and the use of contraception.
[P1] Parents would rather teachers taught sex education. [P2] Therefore, teachers should teach sex education.
[Rejecting P1] Not all parents want teachers educating their children on sex. For those that don't, comprehensive school sex education programs are a direct violation of their religious beliefs.