Marriage is about sex and companionship and, with a few extreme exceptions such as royal marriage, always has been. Everything else, the birth and bringing up of children, financial gains, security, inheritance, honour etc, are the incidentals. Of course, sex (specifically the enjoyment of sex) has evolved in our species to make sure the species continues but couples don't get married to continue the species, they marry for the sex and companionship. If couples married in order to continue the species then human beings would not experience orgasms or enjoy touching each other and intensely sharing their lives with each other. Heck, nowadays women would just go down the AI route.
As Christianity is all about relationship and still holds to the idea that humans have free will, it is Christians who should be championing same gender marriage. It would be more appropriate for atheists to condemn same gender marriage as it is about free will relationship not the selfish gene. Same gender marriage puts the emotional, specifically love, before the mechanistic and practical which again is a Christian way of seeing the world.
Those in the churches who oppose same gender marriage are guilty of buying into the soul-denying zeitgeist of modern, secular, atheistic society. They are functionalists who deny the uniqueness of every living creature. They are stating loud and clear that we are machines and in doing so they are denying the existence of the Christian God who has always allowed his children to choose for themselves and who has always put pure, unconditional love before everything else.