Separation of Church and State, but mostly of Church
It is often asked what harm, if any, it would do to heterosexual marriage if gay marriage were legitimized?
I suppose if we lived in a strictly Libertarian Commonwealth, where marriage was nothing more than a contract enacted for the services (sexual, child-rearing, companionship, and so on) of a willing and fertile young wageearner-ess needing a steady income and a place to stay, the presence or absence of a member of the opposite sex would not matter to the work contract. Libertarians hold that all manners and morals (even those that have negative external outcomes, such as fathering and abandoning bastard children or experimenting with mind-altering drugs) are strictly private, and ain’t nobody’s business what you do, or to whom, or how many. Just get it all in writing first.
However, in the intrusive nanny welfare state that America now is, and in the Euro-socialist failed state America shall most likely soon become, there are no private manners or morals.
In a socialist commonwealth, if a man fathers a bastard child and abandons it, the taxpayers pick up the tab and raise the child. If the man takes drugs and debilitates himself, the taxpayers pick up the tab and send him to the doctor, or to rehab. Everything is a public matter. If Joe the Plumber, tax cow, pick up the tab for your motorcycle head injury, then he, not you, get the say whether you wear a motorcycle helmet, because he underwrite your risks.
And modern socialists are primarily concerned with changing the minds and personalities, the values and attitudes of the subjects of the social engineering experiments.
Socialism started as an experiment in millenarian pseudo-economics, and it made windy promises about being more efficient and producing more wealth than the engines of capitalism. But that is from last century. Modern socialism is social, that is to say, it deals with mind-control. It wants to teach, educate, condition, mold and reprogram human beings.
In its own way, modern Socialism is Aristotlean, because it seeks to carve and shape the subjects of the state, and train them to certain virtues. In Aristotle’s case, the virtues were justice, prudence, temperance, and fortitude. In the case of the modern socialist, the virtues sought are lust (especially perverted lusts), envy (especially class envy), greed (especially greed for the unearned), pride (also called self-esteem), wrath (see the rioters outside World Bank meetings for details) and so on. Where Aristotle wanted men of self-control and Spartan discipline, the modern Left wants the opposite.
Hence, normalizing homosexual marriage, in a modern socialist commonwealth, will impose upon the state a duty (if not a holy crusade) to seek the destruction of any remaining pockets of normalcy and decency. Perhaps in a libertarian commonwealth where everyone minded his own business, perverts and straights could live alongside each other in peace: but in a community where the state has the task of molding the public mind, one must prevail over the other, and mutual toleration is impossible.
Suppose, for example, a Catholic charity that seeks to find homes for orphans wishes to follow its own sacred teachings, and not to place orphans the homes of a gay couples If the Catholic Church gives instructions to her flock to that effect, in a socialist commonwealth, this is no longer a private matter: instead the hierarchy of the Church will be pressured, or commanded, to disobey the Church officers and obey the state.
This is not a hypothetical. It has already happened. The City government of (where else?) San Francisco officially condemned the Catholic church and asked the sitting archbishop to defy church teaching. Below is the document:
http://www.thomasmore.org/downloads/sb_thomasmore/CatholicLeagueSF-CityofSFResolution.pdf
I note with wry displeasure the return of the "Rome is a foreign power" language which figured so predominantly in nativist denunciations of Irish immigrants. It seems those days are not behind us.