It's a tough one. I mean wasn't that exactly what it was like if you go back far enough? At our core being together is what we are, I don't see how if two people love eachother, understanding what that means and requires fully, how it is going to damage a family. It must be better when two people who connect raise a family than two who don't. And while it wont encourage the growth of a family in the long run (no branches out) It still has a chance of continuing a family, legacy and all that being seen as important things during the middle ages when these things really became 'taboo' as people wanted to emulate the kings and queens of certainly Europe leading to the western reasons why these things are taboo.
Incestual paedophilia is another ball park... because then you start asking what is sex... do you see animals having sex with their offspring? Yes, in baboons sex is used to diffuse tensions and as a form of social bonding like playing a game of monopoly (Bad example maybe) Age and relationship is not seen as an issue there. The internet has drawn the human race so close together and for once the most complex organism on the planet is meshing it's collective conciousness together.
These issues are so very delicate. And if not treated right it can get out of hand, realise this is a highly controversial topic... it's not like talking about the next COD or the world cup. This is the type of thing that can get you onto watchlists or at least it will do soon. While the laws are as they are these things will remain illegal, and I do not suggest you break them like people did with interracial or homosexual laws. These things will take time to work out and the fact is society has to get the egotistical 'me me me' notion out of it's head before any of this can be fully realised.
Because while people are thinking 'me me me' sex is not strictly about the other person and therefore that person has to be protected if they are in a 'at risk' group like a child or a relative where it is easier to 'lead them on' for your own personal gratification.
THAT is why we have the laws to avoid anyone being 'used' and 'abused' by those out for their own ends. So even if you do do it for all the right reasons. Because there are people out there who don't and there is no guaranteed sure-fire method of distinguishing between an honest person and and 'evil' one the laws must remain as they are.
We as a species have evolved to teach and educate, to bring up our young to be able to make their own choices in life and live it hopefully to their full potential. However this has been a rapid evolution from small closely knit groups of people where education was sparse and knowledge was just about surviving the winter to a planet spanning society with the universe almost at our fingertips, and this has occurred in a matter of seconds on the evolutionary timescale... the Dinosaurs were around for 180 MILLION years... 'Humans' have only existed for 2 million, modern humans for 250,000 years... from banging rocks together and going ugg to sending men on rockets into space and instantaneous global communication has happened in a mere blink of an eye... it's no small wonder something so central and core to our being and our own identity (sex) has become our most valued and closely guarded issue.
The only other thing we hold in such reference is our own personal power. And THAT is the issue... we hold our Power closer than anything. The ability to affect others is more important to us.
And I'll admit I don't have all the answers. I've spent alot of time thinking about this issue as you maybe able to tell and it'll require a few more years before I'm certain about anything. Some see sex as an instrument to be used, But I see it as being more of a core part of us. And this is where I fall down.