here are many nations and cultures which expect youth to abstain until marriage

And yet kids keep having sex and babies.

The only way these cultures successfully prevent it is by separating the sexes and actively monitoring them. The kids are not different-- the fact that they are closely monitored is what is different.

and even so... that prevention of sex by these strict cultures is still a myth.

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Increased homosexuality and anal sex since it doesn't count.

This is like american teens who think of themselves as virgins when they've done everything *except* having intercourse.

But hey, if it helps you sleep at night- keep believing most humans don't like and don't have sex whenever possible.