“Because the rest of the world does it this way, it must be right!” Doesn’t seem like a valid argument.

It's not supposed to be proof it's true, but it puts this into context. There is a cultural norm of seeing this as some kind of unnatural anomaly, which contradicts historical records and most of the modern world's law.

Once you establish that, what reason is there to say teens shouldn't decide for themselves?

No one contests genocide is harmful, at most people denying a genocide, but it's never questioned that mass murder hurts people. Of course it does. Most of the world doesn't think a consenting teen having sex is harmful, so that is very much a contested issue and not just some fringe group like NAMBLA.