It's not just about underage. It's about power and the ability to use it. There are plenty of people past the age of consent who are manipulated and sexuality abused by those who have power over them, and who find it difficult to come forward even when they think they have been abused because they see it that they are doing the betrayal. And again this is clear in many witness testimonies of such abuse.
Ignoring the power relationships that are there by default in a family is hugely dangerous. This isn't about denying people the right to sex because of any type of a religious view, it's about what is patently clear in the cases of abuse after the age of consent that we have seen on the last fifty years.
Your problem is that you are trying to perfect something that is inherently imperfect, man, humanity, and will prove yet again the aphorism that "what man can invent man can circumvent".
You can make all the laws you like to ban whatever you like and someone somewhere will find a way to get around said laws.
You post ideas which I have no doubt make you feel good for having posted them; unfortunately, these kinds of measures have to be imposed from above, no not by god, and there are two diametrically opposed things which float to the top - cream and scum!
You only have to look at the recent spate of historic sex offenses to see the truth of this statement.