look, it doesn't mean much to me either, i'm no teenager.
Hence the extreme example I gave of the married middle aged man not hugging a female non relative. It's no big deal sexually for them either, but on principle they don't do it.
So, you can ask why if you want.. I think in that situation it's largely cultural. In western culture, once past a certain age, it's no big deal.
but for teenagers that aren't sleeping around, it is not just a cultural issue.