Partly it comes from the weird and oversimplified "sex work" label. That label contains such a wide variety of fundamentally different things that you could have ten conversations about it while never referring to anything that's even remotely comparable to anything in a previous conversation.

Anyway, the point I'm getting at basically boils down to what someone already said in another reply to you. "Sex work" is obviously fundamentally different from other kinds of work, and all you have to do to demonstrate that is ask why it would be wrong for a 16 year old to do it.