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.