If civil unions become "marriage" then marriage itself will become covenant marriage. In some states covenant marriage is already legally recognized. Gays will find their victory no victory at all.
A Covenant Marriage is a Civil Marriage under the laws of the sponsoring state that requires per-marital counseling, restrictive reasons for divorce, and a commitment for the long term maritial relationship.
As a function of law, and in a state where Same-sex Civil Marriage were to be recognized, what would preclude same-sex couples from entering into a covenant marriage under that states laws?
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Because a covenant marriage is the legal union of a Husband and Wife. A man and a woman. The covenant is not between the parties, it is between the parties and God who is actually a third party to the proceedings. A Covenant marriage is religious in nature, specifically Christian. While all marriages and civil unions are presided over by an officer of the court, covenant marriages are only performed by a Christian cleric. There is a requirement of pre marital counseling, but that counseling is done under Church supervision. And the Church itself can decline to perform a covenant marriage for any reason it deems worthy. Gays would be in pretty much the same position they are now. They can have a civil union. If they find a pastor sympathetic to their cause they can get a marriage ceremony.
Marriage today is nothing but a civil union. There is nothing particularly special about who signs the marriage certificate. It could be a Cardinal of the Catholic Church or a clerk standing behind the counter at a county office. In fact, it might end up even worse for gays who now find themselves really excluded. Imagine going to get your wedding cake and get told "Sorry, we no longer make wedding cakes. We only make covenant wedding cakes". Now they have no place to go.