Several centuries ago, marriage was a religious event. After many years, the state and the church gradually separated, they each emerged as a separate entity.
Marriage stayed with the state as a way of the state controlling the people. The church continued to be the record keeper for the state, recording births, deaths and marriages which were permitted by the government.
Marriage continues to be a theological concept to this day.
The people who oppose the marriage of homosexuals seem to be mostly religious people.
Marriage is an issue of religion and the idea of state control of marriage goes to the separation of church and state.
Let’s get the state out of the marriage business, then recognize the contractual aspect of marriage by doing only civil unions.