What is it about our understanding of matrimony that makes the arguments for "marriage equality" seem so plausible to so many?
If we, as a society, still believed marriage was essentially about lifelong fidelity and children, and somebody proposed that a same-sex relationship be regarded as one sort of marriage, it would seem implausible, even unthinkable. After all, such unions are inevitably infertile and notoriously impermanent and non-exclusive.
But we've already downgraded "traditional" marriage to a (usually) long-term relationship between two people who “have feelings for each other.”
Children are an optional accessory which may be acquired the old-fashioned way or by any number of alternative methods.
In this context, calling same-sex unions “marriage” doesn’t seem so implausible at all. Like anybody else, people who experience same-sex attraction may have feelings of attachment, may or may not wish to commit to lifelong fidelity, and may or may not desire children.
Long before “gay marriage” entered anybody’s lexicon, the understanding of matrimony had been revolutionized as to fidelity and procreation.
First, fidelity: Divorce, especially the no-fault kind, was mainstreamed long ago. People began implicitly or explicitly vowing to stay together “as long as we both shall love.” Permanent fidelity became an aspiration, not an essential. It was never dumped outright, just downgraded to an “ideal.”
I think the vast majority of brides and grooms on their wedding day do hope and even expect to stay married for life. But they don’t really make an unconditional commitment to do so, and they know that such a commitment would never stand up in court. A marriage contract is the least legally binding of agreements.
Second, as to children, a vague sense that marriage isn’t complete without them does remain. Spouses who can’t or don’t wish to have any report being pressured and prodded to do so, But viewing the begetting and education of offspring as essential to marriage was kicked out of the mainstream long, long ago.
The idea had been that children--the visible, incarnate expression of a couple's love--should enjoy a stable home and be raised by two people committed to each other (see first point) and to them.
Of course it didn’t always work out that way: death and separation sometimes intervened, though the sheer quantity of breakups and ensuing radical instability is something very new.
Still, the common understanding was that children are one of the two things that marriage is for.
(Much might be said, of course, about the respective weight of the unitive and procreative dimensions of marriage. I don’t propose to address the evolution of Catholic teaching on whether procreation is the “primary” end; rather, I’m noting the disappearance in the wider culture of the idea that marriage is essentially ordered to procreation at all.)
When you've come to see matrimony as an institution essentially about neither fidelity nor procreation, you’re liable to be at a loss when presented with the case for “marriage equality.” Opponents of calling same-sex unions “marriage” talk about the dangers of “redefining” marriage. But that redefinition already happened decades ago: if it hadn’t, this further step would never have gained traction.
We’ve redefined procreation, too. Proponents of traditional marriage point out that same-sex couples cannot beget children. But the prevalence of infertility, IVF, artificial insemination, surrogacy, and blended families with stepchildren and half-siblings has clouded the issue. Begetting a child is seen as just one way of acquiring the legal title of parent to a person with whom you may or may not have any biological connection. "Privileging" biological procreation is seen as denigrating families constituted in other ways.
Even these permutations of procreation are something of a moot point, though. Since children, however conceived, are already viewed as an optional accessory of marriage, what sense does it make to say that two men can’t get married just because they can’t procreate with one another? What does that have to do with anything?
Proponents of traditional marriage often point out the very high incidence of gay promiscuity. Uncontested statistics about its extent are hard to come by, but no one disputes that monogamy is the exception, not the rule.
Whatever the scale of the difference, “traditional” marriage is no longer viewed as being essentially about fidelity. So what sense does it make to say that, because their average number of "partners" is higher, gay people can’t marry each other?
They argue that they’re as qualified as anyone else to enter into a long-term relationship with someone they have feelings for, and they have a point. What they propose is not so much a redefinition of marriage as a next logical step.