Why? Why should the State do more in civil contracts than make sure they are enforeced? If we are to say that anyone can "marry" anyone, why should the state put all the numerous restrictions and obligations on third parties that are not parties to the contract?
You are just stating a position when you state that "marraige is legally a state matter". For many, many years it was not. No marriage licenses were sold in many states before 1900. I am positing a position that marraige should not be a "state matter".
Please explain why marraige should be a "state matter" and if so, why should not the state restrict it?
The reason for the state's continued involvement, i.e. the civil institution of marriage, is still the same. The state has a compelling interest in maximizing the number of children being raised with both a mother and a father who are married. The state does not have much of a compelling interest in gay couples staying together who in any case do not have as many children proportionately, and also are generally inferior parents, simply for not having both genders represented.
That is a good question.
Marriage is truly a personal matter between parties that religious and civil authorities have -- over the course of time -- regulated, with consequent problems such as those presented in the gay marriage question.
I'd say that a secular state dedicated to individual freedom is a best-case power, if any power is necessary, for protection of individual rights in marriage situations.