Having a kid at 16 will screw you up. Most people have a moral objection to screwing up their life like that, as well as the practical objection. Most religions of the world have some form of moral objection to sex outside of marriage, it can be easily argued that the moral objection comes form the practical issues. (most religions predates any form of effective birth control. Today you can argue that the moral is obsolete)

Or put it another way, you cannot talk about kids having sex without everybody bringing their own different morals up. The two are very intertwined.