One of the difficulties faced by anti-marriage pro-discrimination advocates in their attempt to argue that the law should discriminate against same-sex couples in marriage law because “marriage should be limited to those who can procreate” is that even a cursory examination of marriage as understood and practised throughout the world today would throw up many, many childless married couples for whom not having children turned out not to make their marriages meaningless after all.
So they have to try to find a way to distinguish between these childless but married heterosexual couples and childless but MUST NOT BE MARRIED homosexual couples.
And one superficially-appealing way of doing that is by talking about the “kind of relationships” that produce children. Trying to define a broad category so you can ignore the contradictory examples as outliers.
On closer inspection, it doesn’t work at all.
First, it doesn’t explain how it is that we don’t bat an eyelid at childless marriages, which you’d think we would if child-production and child-rearing were an essential element of marriage. Are these marriages real marriages or not? If they are real marriages, then there must be more to marriage than children and the absence of children can’t possibly be a ground to prevent other couples from marrying. If they are not real marriages, then where’s your proposal for the law to start insisting on fertility licences?
(It’s like the polygamy thing – if you’ve seriously got a polygamous proposal in mind, people who keep raising it in order to avoid discussing gay marriage specifically, put it up for us to assess. Because by asserting one “must” follow the other, the slippery slope assumes there’s no significant difference between the proposals, that there are no big problems with polygamy not present in gay marriage. If there are, then we could easily have one and not the other. If there aren’t, then those problems must be present in the gay marriage proposal, in which case COULD YOU TELL US WHAT THEY ARE? If you have to raise polygamy, then the clear implication is that the problems are NOT present in same-sex marriage, and therefore would be a reason to block polygamy even after we pass same-sex marriage. Which of course is why anti-equality advocates will never actually specify what is wrong with the polygamous proposal they imagine – because it would immediately demonstrate why it’s fundamentally different from gay marriage, and implode their “slippery slope” assertion that it “must” follow it. Like the “childbearing is an essential part of marriage” line, the “polygamy will follow gay marriage” assertion isn’t a real argument the people raising it genuinely believe – it’s just there to muddy the waters.)
Secondly, the level at which you draw your “kind of” general category is completely arbitrary. Maybe this Venn diagram will help:
You can say that each of those categories, at different levels of specificity, are “the kind of relationships that produce children”. But it’s entirely arbitrary to draw it at any one level. If you’re going to accept relationships that don’t produce children – which by extending marriage beyond the smallest pink circle you already are – then why stop at the level of gender? If you’re going to include marriages where the participants refuse to have children, and then above that the broader category of marriages where the couples are infertile, then why not include same-sex couples where the couples are infertile?
If child-rearing is an essential element of marriage, then what is the justification for arbitrarily drawing the line at one of these levels and not another?
The anti-equality anti-marriage people have no answer to that, because they aren’t trying to make marriage more about children, they’re not trying to discourage infertile people from marrying unless they’re gay, and they’re not trying to encourage couples with children (which includes gay people) to get married. They are simply looking for plausible-sounding excuses to justify discrimination against gay people they really want for much baser reasons they can’t describe in public.