That's because you can't, given your convoluted logic. Pedophilia has also existed since the beginning of time and only arbitrary law makes it illegal.
How about two consenting adult siblings, shouldn't they be permitted to marry too? That's been around since the beginning of time too. And save the lame genetic defect argument , they can always have an abortion. Either admit incestuals should be allowed to marry too or at least admit you're a hypocrite.
I don't really think those need addressing, they are flame bait,That's because you can't, given your convoluted logic. Pedophilia has also existed since the beginning of time and only arbitrary law makes it illegal.
Actually at least two other posts in this thread have addressed this (one of them is mine), and it doesn't seem hard to deal with at all. If the parties to a proposed marriage aren't legally able to give consent, there's no marriage.
As for incest, well, a limitation on marrying relatives isn't a limitation on the form of marriage, so I'm not sure it's relevant here anyway.
Assuming that they are "consenting adults, free to do what they want", why should we stop them? Well, we have discussed marriage (or civil union) in the view of "the interest of the state". In other words, what does the state (all of us) gain by restricting this behavior, and does doing so outweigh the rights of the individual?
In the case of incest, I would offer that the state is interested in promoting long-term stable relationships and multi-generational family lines. If a brother-sister, or father-daughter, relationship were to occur, that would defeat the purpose. In addition, how would you divorce your brother or cousin, if it were to come to that? They would still be your brother or cousin. You would still be family, and the family cohesiveness would be destroyed, exactly what the state is trying to prevent.
I readily admit that this is not a carefully reasoned argument, you caught me by surprise and I am just typing off the top of my head here, but i didn't want to just leave it unaddressed.