Marriage is Natural Law. Even many animals mate for life - many birds, for instance.
It’s Mother Nature. They want to overturn the laws of nature and whenever that is attempted, the results are never pretty.
And in the rare case where animals procreate without a separate male and female, the animal has the ability to procreate on its own. As far as I know, God (or Nature if some prefer) has not granted that physical ability to humans.