"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." -Manuel II Paleologus
Many of them don't want equal footing. Ask them if they'd be willing to give up any & all of their push for special status if they were allowed to marry. Since they are not physically able to procreate with their partner of choice, I can foresee a potential push for establishing affirmative action in adoptions down the road.
That's the kicker.
Homosexual 'marriages' are against both natural and civil law.
In nature, a species must reproduce in order to remain viable. Homosexuals cannot.
In civil law, homosexual marriages cannot be sanctioned because civil law is based on common law. Common law is rooted in Biblical law. You cannot join two people in Holy Matrimony if that union is an abomination to God.
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Blackstone's Commentaries on the Laws of England, Vol. 1, 1735.
Upon these two foundations, the law of nature and the law of revelation, depend all human laws; that is to say, no human laws should be suffered to contradict these. There are, it is true a great number of indifferent points, in which both the divine law and the natural leave a man at his own liberty; but which are found necessary for the benefit of society to be restrained within certain limits.