Leftist atheists still suffer from the legacy of Christian slave morality. When Christianity began it very much resembled what we call leftism, in fact leftism is rooted in Christianity. You know all that love thy neighbor, we're all equal, help the poor and oppressed, money is evil, hippie bullshit, etc is all found in the Bible and all rehashed by leftists.
For instance:
>"All the believers were together and had everything in common. Selling their possessions and goods, they gave to anyone as he had need."
Pretty much the Marxist motto:
>From each according to his ability, to each according to his need
Engels even admits to this:
>The history of early Christianity has notable points of resemblance with the modern working-class movement. Like the latter, Christianity was originally a movement of oppressed people: it first appeared as the religion of slaves and emancipated slaves, of poor people deprived of all rights, of peoples subjugated or dispersed by Rome. Both Christianity and the workers' socialism preach forthcoming salvation from bondage and misery; Christianity places this salvation in a life beyond, after death, in heaven; socialism places it in this world, in a transformation of society. Both are persecuted and baited, their adherents are despised and made the objects of exclusive laws, the former as enemies of the human race, the latter as enemies of the state, enemies of religion, the family, social order. And in spite of all persecution, nay, even spurred on by it, they forge victoriously, irresistibly ahead. Three hundred years after its appearance Christianity was the recognized state religion in the Roman World Empire, and in barely sixty years socialism has won itself a position which makes its victory absolutely certain.
So leftist atheists while figuring out there is no skywizard are still morally stuck to Christian slave morals. So the thing they cling to is "hey at least we accept gay marriage" like it makes much of a difference.