No, that'd be that the definition of procreative sex is written into our DNA.
Marriage is the sanctification by G-d of the union between 1 man and 1 woman.
And there isn't anything in our DNA that forces 1 man to only sleep with 1 woman. Heck, if we want to pull out 'natural law', then it's in the best interest of a male animal to impregnate as many females as possible, to ensure the continuation of his genes/bloodline. And *THAT* is most definitely *NOT* marriage.
There are different meanings to natural law. Humans are meant to follow a natural law that has choice embedded in it - God given free will. I’m not referring to anything Darwinist or based on evolution theories.
Animals do not have the choice, that free will.
Marriage has always and will always mean a man and a woman. Some cultures, religions and times have permitted polygamy, but that is still man+woman.
It has never been and will never be man+man or woman+woman or any other sicko combination. If humans pretend it means that, they may play at that for a short while, but it is foolish, insane and against both the law of nature in the cells, and the law of nature inscribed in everyones’ hearts, by God.