@weyr
You are actually confusing age of majority and age of consent.
In most countries, age of consent is lower than age of majority.
For example, age of consent in Japan is one of the lowest, at only 13 (while age of majority is one of highest at 20). That's probably the biggest gap in the world actually.
However, this age of consent is limited by the fact the adult mustn't have a relationship of authority over the minor. I guess being the editor could be considerered as an authority over her.
Anyway, this is merely a legal discussion. Ethic is a whole other subject, and I actually don't like at all this idea of an editor having sex with a 16yo teenage girl. Considering his other mangas, the mangaka clearly have a problem with that to me.