First I think we can establish with some certainty that Gertrude Stein's use was not the beginning of this metaphor. Consider the similarities between Stein's "As a Wife Has a Cow: A Love Story" and the Greek fable of IO, who was loved by Zeus and turned into a cow when Zeus's wife, Hera, discovered them. Hera later sent a gadfly to infect IO in cow form, which caused IO to wander in madness across the world for many years. Finally Zeus transforms IO back to human form and she births Zeus's child.
Stein was extremely well read and would have considered all meanings. She would not have overlooked the connection between her choice of words and this Greek story. Certainly she would also have been fully aware of the sexual imagery both in her choice of words and in the original Greek story, just as she would have been aware of the linguistic connections between gadfly and estrum, which is the female version of orgasm. (look up estrual, estruation, estrum)
So on the one hand the analysis that there is a level of meaning in this poem that can be connected to orgasm is correct. It's a very shallow level of meaning and only a step towards grasping the ultimate meaning of the poem, in my opinion. When I consider this poem in connection with the Greek story and with Stein's life I see a lot more.
Full poem is here on pages 481-482: http://www.archive.org/stream/selectedwritings030280mbp/selectedwritings030280mbp_djvu.txt
For instance, if the poem is only about orgasm, why the reference to the fifteenth of October? In Stein's story "The Autobiography of Alice B Toklas" the date fifteenth of October appears in two important ways. First, it is connected to travel, when they are told they can return to Paris. Second, it is connected to "The Fifteenth of November" which is a poem Stein wrote and was trying to get published.
Story can be found here: (edit I'm limited to two links so you'll have to add http:// to the front of this) gutenberg.net.au/ebooks06/0608711.txt
When I look at the poem I see connections to giving birth, as an author gives birth, to a creative work. I also see a lot of frustration in the poem. When I first read it I thought it was like a very droll version of an early attempt at the style that Dr. Seuss ultimately perfected, but once I began connecting a few dots, I thought that what I originally perceived as drollness might instead be frustration. Going back to "The Autobiography of Alice B Toklas" the war is going on and there's a lot frustration connected to that and also TS Elliot asks Stein to write a poem, but insists on a new work, so she produces "The Fifteenth of November" and writes "It was all about wool is wool and silk is silk or wool is woolen and silk is silken. She sent it to T. S. Eliot and he accepted it but naturally he did not print it." I see a lot of similarity in the language that she uses to describe the poem "The Fifteenth of November" and the language in "As a Wife Has a Cow: A Love Story" and I also understand that a writer who is told to produce a new work and that it "will be published" - so she produces it and it's not published - might understandably feel frustrated.
If that writer was familiar with the Greek story of IO, she might even empathize, as if the man who had been a part of her conceiving a new creative work had then turned her into a cow and ultimately led to her having to wander in frustration, while the creative child waited to be born.
She begins the poem with words like "all of it" and "as to be" and "has made" which all ring of the creative process.
Then there comes in the "when he can" and, later, "not and now, now and not" and "On the fifteenth of October as they say, said anyway, what is it as they expect, as they expect it or as they expected it, as they expect it and as they expected it, expect it or for it, expected it and it is expected of it." This last part seems to ring of her being promised by Elliot that the poem would be published in the October issue but it wasn't, and she had provided a new poem, because he had told her he would publish her poem if she produced a new work. So what did he expect? Because a new work was conceived and it wasn't published/birthed, at least not immediately, and again not in October, and how long must she wait? In the end she is still waiting: "My wife has a cow." (Stein being the wife in this creative process)
Of course one could argue that having the cow is giving birth, not still waiting, and it certainly seemed that way to me at first. But in the IO story the time of the cow is the time of frustrated waiting and wandering, not yet giving birth. And I think Stein was just perverse enough to like that the easy meaning is not the correct meaning. What is also very interesting in my interpretation is that if Stein is associating having the cow with frustration then it ties very well to the modern meaning, linked by Stein from the ancient Greek story of IO to the late 1950's hip lingo.