Marriage, and Ethics, and Taboos, and Incest and "Unacceptable Intrusions Upon the Right to Sexual Self-determination"

Is the form we call  "marriage" a consequence of natural law?

Or is the form which is now under assault, one woman married to one man, an artificial construct, either of society or religion?
Those who would so limit the definition are held in much contempt by the chattering class. They.... we, are cruel, hate-filled, small-minded, bigots, homophobic, and most damning of all, "on the wrong side of history."

It seems to me that in order to endorse homosexual unions as marriage one must propose that the old way is just an accident of societal mores, and consequently that any definition of marriage is indeed artificial.

This would not just apply to the sex of the members of the construct, but their numbers as well.
The state would have no basis on which to limit it to "two."

If the civil government can force an adoption agency to agree to place children with gay couples or cease operation, why not with a straight threesome? why shouldn't committees adopt?

And why not with a sister and brother?
In fact, why shouldn't two sisters marry each other?

Laws banning incest between brothers and sisters in Germany could be scrapped after a government ethics committee said the they were an unacceptable intrusion into the right to sexual self-determination.

“Criminal law is not the appropriate means to preserve a social taboo,” the German Ethics Council said in a statement. “The fundamental right of adult siblings to sexual self-determination is to be weighed more heavily than the abstract idea of protection of the family.”

Their intervention follows a notorious case in which a brother and sister living as partners in Saxony had four children together. The couple had been raised separately and only met when the brother, identified only as Patrick S, was an adult, and his sister Susan K was 16.

I don't know the science of it, perhaps long-standing claims that a ban on incest is a matter of health are superstition - the Hapsburg lip isn't such a danger, is it?

But if the most forceful argument against laws which limit the degree of consanguinity acceptable in a legal marriage is that they constitute an "unacceptable intrusion" into a purported "right to sexual self-determination" are any laws governing sexual conduct that takes place out of the public eye wrong?

(Th silly LGBT, or LBTG or whatever it is, community's claim that there is some dissonance or hypocrisy in not allowing gay marriage while at the same time not outlawing zoophilia is is dishonest on the face of it - they like to pretend that they simply are in pursuit of freedom to behave as they wish, but what many really seek is endorsement of their behavior. I don't really care what goes on in your barn, but you aren't insisting that Trigger be your plus one at a family wedding, right?)