I think it will happen very quickly too. But it will be on equal protection grounds. And the courts will ignore the purpose of marriage and all the laws relating to the state of being married, which is to protect the basic unit of society, that is, the family, and thus to protect society itself. However most of those laws are state laws, and they should remain so. Nothing in the Constitution gives the federal government any power in this area.
I won't be surprised if the Supreme Court hears a homosexual marriage case before it hears a second amendment case.