I don't see a better way to do it, but what would make them think that results based on asking teenagers who they are having sex with would be reliable/actionable? This isn't even addressed in the article?
That said, I have no idea if they discuss that in the paper either.
I once amused myself for a few minutes by dreaming up an app for Facebook that would let you claim a relationship with someone and send that person a message allowing them to confirm. It would then collect statistics like false claims, and potentially produce graphs like the one in this article about your social circle.