Marriage is a religious thing, according to social conservatives (Not that I totally agree) So if you give the gays benefits without dirtying the religious institute of marriage, their is no harm done. At least, that’s the justification I’ve been able to piece together, I’d love to hear others reasoning.
Marriage is a religious institution that has existed for thousands of years - before the existence of the US government, before the drafting of the constitution, before european settlers even arrived in the US. To legislate a new definition of marriage destroys the concept of the separation of church and state.