Learning the consequences of your actions can lead you to doubt whatever you will do. Sex education provides the people with knowledge of the aftermath of a sexual intercourse. Yes, others may still do it but some will hesitate due to the possibilities of unwanted pregnancy, sexually transmitted diseases and many others.
Their choice is heavily biased to the knowledge and guidance they had, which is why sex education can decrease, if not prevent, the rate of pre-marital sex.
The teens should certainly be aware of legal and other consequences, but most of them already are. Most of them know that having sex with an underage girl is illegal, that girls will get pregnant and face social stigma, that unsafe sex leads to STD infections and suchlike. I think most teens are aware of that. What should be taught is how to reduce desires. 'In youth, when the physical powers are not yet settled, he guards against lust.' (Analects 16.7) They should be taught that desires ought to be limited, and be encouraged to ignore their desires.
In Europe, sex education has a liberal bias; it is taught that pre-marital sex is normal and acceptable and students are told how to do it, which way they can do it and what protection to use.
Naturally, teaching that X is acceptable will increase the probability of X occurring.
Sex education should be used as a tool to EDUCATE teens on what sex is and the dangers that may come from it. It should NOT be saying that sex is a bad thing, since it is the humans way of reproducing. With this education, teens will still be having pre-marital sex, anyway. We can only hope that because of their education on STDs, that they are using protection and/or taking measures to avoid an unwanted pregnancy, STD, etc.
Sex education was a great way to warn me about what could happen if I wasn't careful with sex, but it didn't stop me from having sex before marriage. If you're smart about it (which most people don't expect from teens) then there should be no problem, I don't think.
Since middle school to high school, they teach the same exact subjects in Health but in different ways. More a majority of sex education is teaching you about the human body anatomy and diseases associated with sex. Sex education in education now mostly teaches you to make sure you have protected sex and have sex at a reasonable age.
Like Philocat said, the present form of sex education is not aimed at reducing pre-marital sex, rather it is aimed at preventing negative outcomes of it, namely pregnancy and diseases. So it gives young people the green light to go ahead and have pre marital sex. It just makes it so much easier and safer to do so. Sex education indulges them.
The focus of sex education changed because abstinence only education didn't seem to work that well, which begs the question why? We know controlling desires and warding off temptation is a tough thing for young people do, especially when everyone else is doing it and media glorifies it, so that's one reason why. Another reason and I think the real reason, is that young people are not taught the importance and value of family or they don't give importance to it because they either come from broken families or are unwilling to be tied down to a family or just never gave it a closer look. Sex has been wholly separated from the fact that it's purpose is to bond two people together with love and mercy between them, for the creation of a family . Sadly, that's because families are not the ideal ones anymore.
Let sex education continue. But if we really want to decrease pre-marital sex, teach the importance of family life and instill in them a desire for those emotional (rather than just the physical) bonds one develops within a (stable and healthy) family and the wish to try and create their own someday with the right person.