Ian Robinson is President Emeritus of the Rationalist Society of Australia and a former editor of their journal, the Australian Rationalist. He writes in Online Opinion, an Australian e-journal of social and political debate:

... The norm for human beings is sexual reproduction which requires not same-sex but opposite-sex unions. At the most basic level, our survival as a species requires the coming together of male and female gametes.

In human societies the way this essential union is symbolised is in the institution of marriage. This is how the centrality of the male-female partnership is celebrated in our culture and, in a non-religious sense, it is sacred; that is to say, heterosexuality is so important to our survival, so fundamental to the continuation of the species, that we have an ingrained sense that marriage as a heterosexual union should not be tampered with. It symbolises in the social sphere the vital role that the male-female gamete union plays at the biological level.

Hat/Tip: Family Scholars