If he could do that with a female roommate why shouldn't he be allowed do that with his male roommate? It isn't reasonable to object to gay marriage on a technicality like that while also being fine with the laws regarding straight marriage.
Didn't I already give the reason? Because it could be used as a loophole in the tax law, that men and women in temporary relationships didn't exploit, even if divorce is instant and the high earner can keep all the wealth that had been taxed at a lower rate. (Because women generally don't let themselves get used like that.) On the other hand, in a community property state, with no pre-nup, that loophole is closed.
It's also generally reasonable, as a structural concept, to object to the idea that laws should treat men and women equally, or men/women and men/men pairings, as some kind of universal principle. California's legislature adopts this stance: it outlaws same-sex boards (at least all-male ones) in publicly traded companies.
Similarly, it's reasonable to examine the question of same-sex relationship laws with more nuance than saying everything should be the same as marriage because equality. Maybe the divorce and taxation laws should be different, because the straight ones ought to be written with children and rapidly declining female fertility in mind. Maybe the "next of kin" definition or inheritance laws should be adjusted, when it comes to extended family. Maybe gay marriage should have a fine-grained series of check boxes to opt in to each of the privileges and duties and consequences, because that's just good customer service for gay citizens, but straight marriage should be all or nothing, for good reasons.
I personally think that this is outdated, and that we should incentivize such things in more direct ways if we should at all, but this is a reasonable point of disagreement prima facie.
Straight couples can get married without any plans to have a child. Why is is different for gay couples?
Gay couples can also have a child either through adoption or biologically with a surrogate or sperm donor.
And at the same time, it is true that a gay couple can have children through adoption, or biologically.
It is for both of these reasons that I disagree with the argument.
That being said, at the time when the argument was relevant, the overwhelming majority of married heterosexual couples did have children, and gay adoption as well as IVF were not really a thing.
But nowadays, of course, anyone making this argument would be making it from bias. In the framing of the thread, which was the 1940s, it is more reasonable.
There is also the scientific question of whether sexual orientation is innate or a preference. For a long time, the science was wrong. Key research was done on that in the 1990s and early 2000s, and that filtered into the public conscience over time. Even as late as 2013, only 40% of Americans recognized that sexual orientation was innate. But once that premise became accepted, support for legalizing same-sex marriage quickly followed.
In the context of the US, your arguments are compelling, but they show exactly why the issue isn’t “morally indisputable.” It’s worth reading carefully through Obergefell. It spends a great deal of time establishing several things. First, that sexual orientation is innate and immutable: “Only in more recent years have psychiatrists and others recognized that sexual orientation is both a normal expression of human sexuality and immutable. See Brief for American Psychological Association et al. as Amici Curiae 7–17.” Second, that the understand of marriage had in fact changed to deemphasize children. Third, that new studies had shown that large numbers of same-sex couples were in fact raising children together. All of these established why same-sex unions should be protected as part of the fundamental constitutional right to marriage. Other approaches were possible and had been advanced. Some on the left wanted a broader rethinking of traditional marriage as a social institution. Obergefell could have treated marriage as just some government tax benefit, which was the libertarian view. Instead, it doubled down on the fundamental importance of traditional marriage, and the government’s legitimate interest in recognizing and promoting that institution, while expanding it to same-sex couples. It was very much the product of political debate and compromise amongst a variety of positions.
And in the end, this debate was ultimately a good thing because the political process fostered understanding and consensus. Same-sex marriage has become accepted quite rapidly. (Abortion rights, by contrast, remain highly contentious and no consensus has ever developed.)
This is rather an argument for a change in taxation laws instead of gay marriage.