Marriage is one of the places in American society where the boundaries between church and state are completely ignored. Clergy are given the power of the state to create legally recognized civil unions.
When the clergy say, "By the authority vested in me by the state of Texas, I proclaim they are now husband and wife," we claim a civil authority that should not be ours to claim. Our authority as ordained clergy comes from God through our governing religious bodies. As a pastor, I don't want any authority granted me by the state of Texas.
In most countries, people who wish to be married go to the courthouse to create their civil union. If they are Christian, they then go to the church to have that union blessed and to affirm their commitment through the language of faith in a Service of Christian Marriage.
This practice points to a proper solution regarding same-sex marriages. All people who want to live in committed relationships should have the right to legally recognized unions, regardless of their sexual orientation. Monogamy is a good thing for society and for those who live within the freedom of covenant relationships. Leave it to religious institutions to determine whether or not they want to bless such unions. But do not give religious institutions the right to determine whether or not the unions can be created.