And I think that expectation that teens "shouldn't" have sex conflicts with the reality of the situation. Too often the same people enacting and voting for these laws are also in favor of abstinence only education, which has been proven to not be an effective deterrent for teen pregnancies. "Don't drive" isn't a useful piece of advice for avoiding automobile accidents. "Don't get on the Internet" isn't a useful piece of advice for avoiding computer viruses. "Don't have sex", likewise, isn't useful for teaching hormonal teenagers how to deal with their budding sexuality.
If anything, creating a taboo around sexuality makes it more enticing to rebellious teens.