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Marriage is a legal institution that just so happens to share the same name of a religious institution.
Marriage is based on sexual dimorphism and community structure. Been that way forever. For all of history marriage has had a relationship with religious beliefs and cultural practices.
Soo many religious laws, practices, ideas, morals, beliefs, and traditions were integrated in the legal institutions and governments of the world. When given a choice, that is the direction that human beings chose to go in.... as religious laws offered the best guarantees about our longevity and moral foundation as a human race. So it doesn't Just "happen" to share the same name, it's just what it is. There is no complete separation of church and state, and the state wouldn't exist without deeply religious racist founding it on the religious backs of others. There is the idea of separation, but it's not like every law, idea, and politician aren't going to be influenced by religion.
Anyway, marriage is what it is. Not what we are trying to make it be. Civil unions with equal legal rights as marriage, should have been what gays fought for.. so that could actually be fighting for something, instead of fighting against the common idea of something.