There is an argument against comprehensive sex education that basically argues that since we do not teach young people how to use drugs safely, we should not teach them how to have sex safely/use contraceptives/etc and instead should focus on abstinence alone. This argument fails for several reasons.
Humans are animals, are biological organisms. Humans have a natural, evolutionary tendency desire to have sex; indeed, if we didn’t, we’d be extinct. Do humans have a natural, evolutionary tendency to do drugs? Certainly not tobacco and cocaine, as those are native to the Americas. Similarly, cannabis is native to central Asia north of the Himalayas. Basically, do not have any biological need to use these drugs. (Medicine does not count; is is possible, although painful, to never use a painkiller, for example.) Alcohol may be a partial exception, as light consumption may have some beneficial effects, but otherwise biology does not make us have to do drugs. Basically, people normally have a sex drive, but do not normally have a “drug drive”.
Similarly, there are different things involved in using drugs versus having sex. How does one go about, say, obtaining marijuana? One can grow it, or buy it from someone. For heroin, one has to buy it. The same applies for other drugs. This allows multiple means to go about and prevent drug use. Besides telling people not to use it, one can also go after suppliers, dealers, and the like. One cannot do that with sex. All one needs to have sex is someone else willing to do it with you. The only way to the same to sex as is done to drugs would be to prevent people from spending time with others. In short, there are generally more steps involved in doing drugs than in having sex.
Finally, most drugs (caffeine is a notable exception) are illegal, and alcohol and tobacco are age restricted. Although there is an age of consent for sex this is not the same thing. Once one reaches the legal age of consent it is legal for them to have sex with someone else willing to and who is also of the age of consent. Even if there is a blue law banning non-marital sex, such laws are unenforced (the government has better things to do than control people’s sex lives). Hence, even if they are de jure illegal, they are de facto legal. This is not the same as people getting away with drug possession. Even if most people who smoke marijuana are not caught, people are still charged if found to be in possession of it. The same happens for other illegal drugs.
Hence, for these reasons, the “we do not teach young people how to use drugs safely” argument against comprehensive sex education does not work.