Why do (some) people consider “The Heat Death of the Universe” to be science fiction?

Question

One of the stories in the collection Daughters of Earth: Feminist Science Fiction in the Twentieth Century (Justine Larbalestier (ed.), 2006) is "The Heat Death of the Universe" (Pamela Zoline, 1967). The story was originally published in a science fiction magazine (New Worlds); and in Daughters of Earth it's followed by an essay that purports to explain why it's science fiction, but I don't really understand the argument there.

Why do (some) people consider this story to be science fiction? (What definition of "science fiction" are they applying?)

For those who haven't read the story, Wikipedia summarizes it thusly:

"Heat Death" is structured in a loosely encyclopedic style, with 54 numbered paragraphs narrated in a deliberately matter-of-fact third-person voice. It centers on a day in the life of middle-class housewife Sarah Boyle as she goes about preparing her children's breakfast and organizing a birthday party. Boyle's domestic sphere is presented as a possibly closed system analogous to the universe itself, and Boyle as subject to the ravages of literal and metaphorical entropy. As the narrative veers back and forth among scientific explanations, descriptions of household events, and philosophical speculation, the cumulative effect is of a mind and a culture on the verge of collapse.

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Edited to add: Since some have questioned my statement that some people consider this story to be science fiction (and the only answer so far basically takes the stance that no one really does), let me elaborate on that a bit.

In Daughters of Earth (which is where I read the story), each story is followed by an essay about it. Conveniently, the essay after "The Heat Death of the Universe" is available online, so you can read it for yourselves; but here are some snippets that show what I'm referring to:

Ketterer’s reading suggests why many women writers have been denied admission to the science fiction camp: woman’s work simply doesn’t merit attention, […]

The debate about whether or not Zoline’s story is science fiction is particularly puzzling in that “Heat Death” is imbued throughout with science fiction thinking.

In other words, the essayist (Mary Papke) not only feels that the story is science fiction, but that it is obviously science fiction, and that people who don't see it as such are apparently male chauvinists.

And it's not just her; the collection's editor, Justine Larbalestier, writes in the introduction (also available online) that:

[…] even sf readers with a less elastic understanding of the field will find that the stories in this collection meet most people’s definitions.

(and lists "Heat Death of the Universe" in the group that "treat directly with scientific theory and practice"), adding that:

Two of the stories have had their science-fictionness questioned: “Heat Death of the Universe” and “What I Didn’t See.” The essays about them by Mary Papke and L. Timmel Duchamp make compelling cases for their inclusion within the genre of science fiction—and more precisely, within feminist science fiction.

(FWIW, I did find Duchamp's case for "What I Didn't See" to be pretty compelling, or at least, it made sense to me. But I couldn't get any purchase on Papke's argument.)

Answer 1

"The Heat Death of the Universe" was first published in New Worlds, which at the time was edited by Michael Moorcock, who was interested in Zoline's visual art as well as in experimental fiction (Papke, 2006). Moorcock, then and now, has often been critical of the restraints some people place on the genre of science fiction:

[...] I hated the endless discussions of what to call various divisions of supernatural or science fantasy and refused to join in. In my view books should be classified according to whether they’re fiction or nonfiction and by author. I know many readers who broadened their reading because they picked out an author in mistake for another.
-LA Review of Books, 2013

As an editor, Moorcock always wanted to publish more than just science fiction, including articles on real science:

I was allowed first choice and to some peoples’ surprise chose New Worlds. I felt there was more I could do with the title. I wanted a large size magazine on art paper so I could publish contemporary painting and sculpture as well as scientific features to produce a blend of art, science and fiction. Compact told me they couldn’t budget for anything more than a paperback size on fairly pulpy paper!
-Amazing Stories, 2014

Moorcock was surprised by the difficulty of finding and publishing the unconventional new fiction he championed, as well as by its lukewarm reception:

I soon realised there were not many writers out there ready to produce the new kind of fiction I visualised. I had to proceed slowly to develop not only the fiction I wanted but also the kind of readership I needed. Much of the early work in my New Worlds was fairly conventional, if aspiring to a slightly more ambitious level of writing. Gradually new writers began to emerge and old ones became increasingly ambitious... Rather innocently, I had thought most SF readers would welcome the idea of a new kind of literary fiction coming out of science fiction! Fandom, at least, didn’t.
-ibid.

Despite the backlash against his ideals, he believes that bringing unconventional, untraditional elements to science fiction was good for the world of literature:

My main interest was not so much to change SF as to use SF to improve what fiction could do. Contemporary ‘litfic’ was bogged down in decayed modernism. SF had developed alongside modernism, occasionally borrowing its techniques. Now so many SF methods are used in the ‘mainstream’ that it seems everyone benefitted. We brought new techniques and improved levels of ambition to SF and SF did the same for literature in general.
-ibid.

In his embrace of the common ground between literary fiction and science fiction, Moorcock found Zoline's story to be particularly significant and affecting:

Pamela Zoline's story The Heat Death of the Universe links the modern myths of science (entropy, etc.) as they are understood by the layman with that great myth figure of modern fiction, the Victimized Domestic Woman. This story, incidentally, struck me with such force when I first read it that I cried. I've rarely been so moved by a story submitted to me.
-from the introduction to Best SF Stories from New Worlds 3 (1968)

In short, this story became prominent in science fiction because Michael Moorcock published it, and went on to champion it as a great story. He did so for several reasons:

• He wanted a place in his magazine for art and science in general, as well as science fiction.
• He believed that science fiction and literary fiction were a continuum, not exclusive categories.
• He thought it was an excellent story because of its examination of "modern myths", and he was quite affected by it personally.

Given his creative and editorial priorities, Moorcock has always refused to define the category of science fiction, or to establish firm criteria to qualify a story as science fiction. I believe we can assume that the story remains well-regarded today because plenty of people agree with Moorcock.

The story also has both resonance and longevity due to being a story by a woman, with a woman as lead character, incorporating feminist social issues, written at a time when these aspects were uncommon in science fiction. It was repeatedly included in anthologies in the decades after Moorcock published it in New Worlds (Papke, 2006), and we may conclude that these anthologies' editors were unconcerned with its strict qualification as SF.

Among fans and editors concerned with feminist science fiction, there has often been an attitude similar to Moorcock's - i.e. that the hidebound rules classifying genres of fiction are not so important. Donna Haraway, writing about science and feminism in her influential "Cyborg Manifesto", stated "the boundary between science fiction and social reality is an optical illusion" (Haraway, 1985, revised 1991). Writing the "Whys and Wherefores" of FeministSF.org in 1995, Laura Quilter declined to construct a strict definition of the genre. She cited diverse definitions, adding up to a field that's as unrestricted as possible, accommodating for anything from quantum physics to magical realism. In 2009, in response to criticism of women in sci-fi, Annalee Newitz wrote "The movement to "feminize" SF has resulted in an attenuation of what science fiction means," but she dismisses this attenuation of the genre as unimportant compared to the material concerns of feminism.

This attitude in feminist SF is concordant with Moorcock's original editorial intention, in that it disregards the "SF vs. non-SF" distinction. While Moorcock prioritized literary merit, many feminists in science fiction prioritized social realism and relevance. In either case, the result was the embrace of writing that was seen as meritous, without handwringing over whether it constitutes science fiction.