
If sex is something that's purely physical with no mental, emotional, or societal contexts or consequences, sure, we can go with "nature" (though this also has some really distasteful implications, if you really dig into it). But in a world where sex doesn't exist in a vacuum and there are things like power imbalances due to age, social status, wealth, etc., and different levels of emotional/mental maturity besides, relying purely on whether or not someone's hit puberty just doesn't cut it. There were imbalances in the pre-1800s and there are imbalances today. In that way, I think the "what nature wants" argument is just a little bit beside the point.