Gay marriage is illogical
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.