Most of the rights and privileges that go with marriage are, in fact, ways to help spouses support each other. If marriage was so "sacred" they'd also be pursuing the outlawing of heterosexual divorce. There was even a time when not one country in the world had legalized same-sex marriage! Married couples are thus much better off than unmarried couples. Children are already being born to, adopted by, and raised by such couples in increasing numbers, and not just those who are legally married.
All of these have been opposed by the same religious and political forces which currently oppose gay marriage.
Marriage gives relationships the ability to grow stronger and deeper. On the contrary, there are good reasons to think that legalized gay marriage can benefit us all. From Our Partners.
The natural world didn't create marriage, humans did.