God invented marriage, not man. God designated marriage, not as a contractual agreement, but as a covenant. Governments may be able to regulate contracts, but a covenant is based on the laws of God and not man. Thus marriage is an institution outside of the bounds of government.
As with all things Statist, the secular State’s definition of marriage and divorce has come to have more weight in society than God’s definition.
God defines marriage as being between men and women. While many men whom God called righteous in the Bible practiced polygamy, Jesus is quoted in Matthew chapter 19 that from the beginning of humanity, it was God’s intent that marriage would only be one man and one woman. In Mark chapter 10, Jesus stated that God, not man, joins the husband and wife and the two are “one flesh”, an act that by its very nature can only be heterosexual.
God defines sexual relations between people of the same sex as against His law, that is immoral. In Revelation 22:15, a book dictated to John by Jesus, God tells us that anyone who practices sexual immorality will not be granted eternal life.
Maybe it is time to get government out of the marriage business and to return it to the private sphere. The problem for me today is that people who want to call themselves “married” against God’s law (as I read it) are willing to use the State to force me to recognize that marriage, which I cannot do. The want to have the State’s public education system indoctrinate my children that homosexual marriage is “normal”. They want to force me to subsidize the homosexual marriage in the tax code just like the godly marriage is subsidized, and they will use state agencies to punish me for “discrimination” if I decline to accept their status in any way.
If the State must force me to acknowledge its power to declare two men to be “married”, then I must support efforts to remove that power from the State. If people who don’t want God defining their personal morality demand a separation of church and State then let us also have separation of marriage and State as well. If those people don’t want any displays of the Ten Commandments in government buildings, they cannot hide behind the Commandments that protect marriage when it comes to “marriage” that God cannot and will not sanction.
The idea that a woman should only breed with one man, who will then assist in the protection and care of resulting offspring, predates both government and religion (even if one accepts Genesis, I don't think it claims the continuous existence of religion between the time of Adam and Moses; if a proper religious tradition already existed, it would not have been necessary for God to instruct Moses in writing the Torah).
Religious arguments against marriage are counterproductive in talking with anyone who does not accept the religion in question. The secular argument is much stronger: 99.99999% of all people who have ever lived, lived in a society which recognized a concept of marriage involving exactly one male as different from any kind relationship involving any other number. The requirement that marriage involve exactly one male is not imposed by government, nor by religion, and the only way it can be viewed as "bigoted" is if the vast majority of people who have ever lived are bigots.
Further, even if gays have the right to call themselves whatever they want, that does not imply that anyone else has any obligation to honor such declarations. The right of free association implies the right to honor the kind of marriages that has essentially always been nearly universally recognized, without incurring any obligation to recognize so-called "marriages" which fail to meet one of the most basic and universal requirements of marriage (that it must involve exactly one male)
That isn't going to happen, it can't happen, and saying so won't prevent homosexual marriage, and is a waste of time, if not a deliberate diversion, and a tactic of libertarians.
If you oppose it, then elect politicians who will pass laws against it.