A lot of this problem is from the lack of polite social settings for young people to have relaxed association. School does NOT count, as it is neither relaxed nor social.
As any dog owner will tell you, if a dog is not able to freely associate with other dogs while it is growing up, it will not know how to behave around them and be a problem—either scared, asocial, or aggressive—likely for the rest of its life. Should that much more be expected from children, who should have even more complicated social relationships?
The truth of this can be found by actually asking children one question: “When was the last time you just sat down and chatted with someone of your own age, of the opposite sex?” For a lot of children, the answer is “never”. Always they are under pressure, there are other things going on, no opportunity to just hang out and make small talk. If anything, they are kept apart based on their gender.
The correct answer is that they should do so regularly. They should feel as relaxed talking with the opposite sex as they do their own sex. Neither scared, indifferent, or aggressive.
As it is, lots, perhaps the majority of children are barely able to see the opposite sex as human, much less as someone they will eventually want to be emotionally close to. This leads to all sorts of social problems down the line.
Strange as this may sound, I don’t think that the problem with casual sex is the result of keeping boys and girls apart. Call me naive...