Oh, you say you're quoting 'nature', but nature didn't invent marriage.
Yes... In fact Nature did invent marriage.
Ya see, Nature provides for every aspect of marriage... First it gives the life, this established through the design intrinsic to human physiology. The hormonal drive to engage in behavior that promotes procreation through the joining with a male through coitus. The requirement for the male to procreate, the the defenseless nature of the female during gestation, thus the need of the female for the protection of the male... further nature provides for the distinct traits inherent in the personalities of the respective genders, which are applied throughout the raising of children, nurtured by the female, trained by the male.
Now I realize that you 'feel' that marriage is a legal contract, which serves to acquire stuff and temporal federal privileges and considerations. You should be advised that such is what is actually known as 'incorporation', wherein any group of individuals join toward whatever purpose they choose, without regard to the number of people or the genders of any individuals at issue.
Of course, such in no way offers any sense of legitimacy, and in no way would produce a popular inference that people of illegitimate nature are anything but what their provides establishes them as. Which means that those of illegitimate nature will natural eschew such otherwise appropriate institutions. They will prefer instead to attempt to establish the elusive legitimacy they crave, through deceit, fraud and ignorance, glomming onto and forcing themselves into institutions which through their established standards enjoy intrinsic legitimacy. Which sadly, given their intellectual limitations, and their sociopathic tendencies, they'll possess no means to recognize that the instant the institution drops the standard that precludes them, the legitimacy otherwise inherent in such evaporates into the ether. Leaving the institution without meaning of purpose and the purpose it served, unserved and the culture that depended upon that service: TOTALLY BONED!
Of course, a spike in socialism and homosexuality is a harbinger of catastrophic societal collapse, which inevitably results in the near extinction of all homosexuals and their most species socialist comrades.
So... I wouldn't spend much time worrying about it.