Sex and drugs are very destructive. They are the signs that a teen has lost control. Although many teens experiment just to experiment, some use sex and drugs to act out because of low self-esteem or problems socializing. They can be considered a cry for help.
I think today's culture has the worst influence on our teens. This culture makes communicating via technology so prevalent that teens forget things such as written language, body language, wand facial expression to name a few as valid means of communication. This is a culture where many kids are not taught to walk away from a fight, but are in many cases encouraged by their parents to seek out fights. This is a culture where many kids feel free to cuss out teachers in class, and then the teachers are blamed by the parents for picking on or bullying their child.
I would argue that peer pressure is seen as the worst influence on teens. I think that peer pressure is seen as the thing that causes teens to do things like having sex when they shouldn't or like taking drugs when they shouldn't. In addition, it is peer pressure that is blamed for other bad or dangerous things that teens do.
Teens do certain other things that are equally as bad as sex and drugs. They might binge drink, for example. They might get drunk and then drive. They might commit crimes. All of these things (according to society) are caused at least partly by the pressure they feel from their peers. Peer pressure is also typically blamed for things like premarital sex as teens feel that they will be left out if they do not engage in sex.
So I would argue that sex and drugs are seen as two symptoms of what is wrong with teens. But the real cause according to many in society is peer pressure.