Most 16 year olds are not mentally mature enough to marry or have kids..
This all comes down to the idea of good faith and cultural pressures. If a man is acting in good faith and is a good man, like Sanji is, there is no problem.
People were fine to marry younger in the past because if you were a bad faith actor, your society would shun you, also people would die younger, work younger marry and have children younger.
Now women are expected to enter the work force at 22 after 15 years of schooling, so the timelines are obviously different, but the arrested development of modern people isn't "natural" by any means and is probably a cause for depression in many.