if you define what's natural to mean what exists, then everything is trivially natural
trivial but important point to realise, and the only indisputable response. Once accepted you can have the debate understanding the distinction is very ideological, focused on desired outcomes, hopes and fears and like you say the main question of what does it mean to be human? Does this thing, this behaviour threaten my idea of what it means to be human?
The natural also spans a lot more, so more simple stuff like what is common, what is ancient, what is predictable, increasingly what occurs without human interference?
but underlying the word is the acknowledgement that somewhere along the line, the fall, we fucked up badly and are spiralling out of control. Unless we get back to nature of course!