Although you can successfully argue the religious ties to marriage, at its core marriage is a state thing -- not a religious thing. Otherwise government could say nothing about it and could not issue marriage licenses (nor the benefits thereof)
As an atheist, marriage has both emotional and practical value. At its core, it is the ultimate celebration of love and commitment to my designated female accomplice

. Practically, marriage provides legal securities with regard to inheritance, insurance, possession, contracts, taxes, among other things.
I can see no reason why any atheists would advocate against marriage as it stands unless it is to separate marriage from the government and hand it to churches, thereby turning marriage into nothing but a useless ritualistic ceremony, and then give government the power to distribute civil union licenses in its stead, in order to disallow any arguments for discrimination.
But that's just me
