I'm not disputing anything you said, I'm just adding on that "well, if they just don't know what sex is" is a demonstrably false presumption. And sure, teenagers tend to not worry about the future as much, which only means that planning works better. If they might have sex and have condoms they will be far likelihood to have sex with condoms than either not having sex because they don't have condoms or stopping in the heat of the moment to get condoms.