The traditional institution of marriage is one man and one woman. The states' current definition of marriage is one man and one woman. The states support traditional marriage to protect the many children that result from sexual coupling. Why should that change?
Whose "tradition" are you referring to?
Should city employees be able to receive spouse benefits from such arrangements?
What about private employers, can they choose to support or ignore "same sex spouses"? Could they face discrimination lawsuits?
What about the inheritance issue? The courts are involved in that.
Can a church refuse to perform a same sex "union" ceremony? Would there be a media backlash against any church that dared to follow this biblical teaching?
You can "say" that you married the Universe but you will be unlikely to push for "spouse benefits/rights". Same sex partners want legal recognition (also remember that spouses cannot be compelled to testify against each other).
Sure are a lot of legal reasons for the government to be involved in determining what is a marriage and what is not.