Sorry, wife, but it’s sayonara, baby. It’s been really great for the last 30 years and I thought it would last forever. But if Ireland follows France and 11 other countries and legalises gay marriage, I’m out the door. It will all have been a sham. Our sons will have to be written off as mistakes. Our whole marriage will have been fatally undermined because the institution itself will have been reduced to a paltry thing.

This, in essence, is the primary argument against gay marriage: that recognising it will diminish all marriages. And it is a stupid argument. A marriage freely entered into is a personal relationship. It stands or falls, endures or collapses, is a heaven or a hell, solely because of the way the people in that relationship treat each other.

The nature of my marriage does not change in the slightest if my neighbours are cohabiting or my workmate is in a civil partnership. And it won’t change when, sooner or later, my gay and lesbian friends have the same freedom to marry (or not to marry) that I had.

I know we’re all supposed to be civil and respectful about this, but let me be honest: I don’t respect the arguments against gay marriage. I don’t respect them because I don’t think they are really arguments at all. They are a pseudo-rational veneer on irrational prejudice. They are so weak that I don’t think any intelligent person would believe them unless he or she had already decided that gay men and lesbian women are not deserving of full human equality. And that decision isn’t respectable – it’s vile.

As well as the proposition that marriage equality would undermine all marriage, the anti-gay argument, as elaborated by the Catholic bishops in their submission to the constitutional convention, consists of two other absurdities. One is that same-sex marriage is impermissible because marriage is all about a man and a woman making babies: “Marriage is the community formed by a man and woman who publicly consent to share their whole lives, in a type of relationship oriented toward the begetting, nurturing and educating of children together.”

If the church believes this, it should refuse to recognise the marriages of those who do not have children and those whose only children are adopted. For the only thing that rationally excludes same-sex couples from this definition is the begetting bit. Same-sex couples who have children, in fact, should be accorded a higher status than heterosexual couples who have no children at all.

The other argument is that opening up marriage to same-sex couples would be unjust towards other kinds of couples who cannot get married. The bishops tell us: “An unmarried adult may share a home with an aged parent and care for them; this is a demonstration of love and commitment but is not akin to marriage. Similarly, two siblings may live together, pooling their financial resources and sharing their interests; this too demonstrates love and commitment but is not akin to marriage.”

Would it be possible to miss the obvious point by a wider margin? The adult caring for a parent and the siblings who live together are recognised by the State as belonging together in the same family. This is all that same-sex couples who want to marry are asking for – not something different, just the same right to form a family and have it recognised.

These arguments are so thin because they are mere expressions of bigotry. You can always recognise prejudice because one of its classic features is the way it places those at whom it is aimed in a double bind. They can’t do anything right. If Jews got rich, it proved they were bloodsuckers. If they stayed poor, it proved that they were leeches. If blacks submitted to slavery, it proved that they were naturally servile. If they revolted, it proved that they were savages.

This is exactly what the opponents of same-sex marriage do to gay men and lesbian women. If they have sex outside marriage, they’re promiscuous and a danger to society and morality. If they demand the right to be recognised as a faithful and eternal couple, they’re undermining marriage and are therefore a danger to society and morality.

There is only one way out of this double bind – don’t have sex at all. This is the official position of some of the Christian churches, who accept that homosexuality is natural and therefore God-given and then insist that God did not intend it to be expressed in any physical way. This makes God a sadist, but it also makes those who hold this view bigots. For at its heart is pure revulsion.

Opposition to same-sex marriage is opposition to the proposition that all citizens are entitled to equal treatment by their state and its laws. And it’s time we called that opposition what it is – not conservatism but prejudice. It is prejudice in the same sense that hatred of blacks or Jews or Catholics is prejudice. And it is no less shameful.