What? A marriage license is a contract? Would you be so kind as explain how you came up with this idea?
And while you are at it, could you also tell us how homosexuals are being discriminated against? Homosexuals can marry if they so desire, as long as they marry someone of the opposite sex.
But what you fail to understand is, the state does not have to allow anyone to marry if they don't want to. You see, marriage is not a right. It is not mentioned in the Constitution. In fact, it was left up to the States to decide, if you have read the Tenth Amendment. Furthermore, in 38 states, there is now a Defense of Marriage Act in place that firmly establishes the definition of marriage as being a legal union between one man and one woman. To date, this has not been challenged by anyone, in any of the 38 states, because of the aforementioned fact.
What the Mayor of San Francisco is doing is in clear violation of state law. He knows it, we know it, and those marriage certificates he has been handing out are not worth the paper they are printed on anywhere outside of the city.