But one factor is left out. IIRC, the number of female converts to Mormonism greatly outnumbered the male converts, so within the Mormon community the male to female ratio would have been quite unbalanced. Monogamy for them would have meant far fewer women bearing children.
So the end effect was NOT fewer children per woman, but more, since under strict monogamy, most of the women would not have found husbands and would not have had any children at all.
What was your source of this myth?
Consider the UK/European converts. Single women indeed made the journey alone, but not in greater proportions than single men.
I've encountered some contemporary Mormons who assumed that there was some glut of widowed women and that therefore, men just had to “step up” and “marry” them as a “plural wife.”
According to the Changing World of Mormonism by Sandra & Gerald Tanner ( pp. 224-225: [LDS} "Apostle John A. Widtsoe stated: ’We do not understand why the Lord commanded the practice of plural marriage.’ (Evidences and Reconciliations, 1960, p.393). One of the most popular explanations is that the church practiced polygamy because there was a surplus of women. The truth is, however, that there were LESS women than men. Apostle Widtsoe admitted that there was no surplus of women: 'The implied assumption in this theory, that there have been more female than male members in the Church, is not supported by existing evidence. On the contrary, there seems always to have been more males than females in the Church.’.. The United States census records from 1850 to 1940, and all available Church records, uniformly show a preponderance of males in Utah, and in the Church. Indeed, the excess in Utah has usually been larger than for the whole United States, ... there was no surplus of women'” (Widtsoe, Evidences and Reconciliations, 1960, pp.390-92," as cited in Changing World, pp. 224-225).
So you're not going to argue with a late-19th century Mormon "apostle" who was writing Mormon material when polygamy was still being openly practiced by Mormons, are you?
Imagine single men having to “do without” such a wife because some men were “hoarding” them, 27, 40, 57 at a time.
B. Carmon Hardy, in his book, A Solemn Covenant: The Mormon Polygamous Passage, says: Because of secrecy surronding such unions, suitors were confused as to who was and who was not available for serious courtship...One young woman told how, as late as the 1920s, she was repeatedly approached at church dances by married men who wanted her to be their plural wife. (p. 321)
Here was a young woman being approached over 30 years after the "Manifesto"...plus think of how difficult it was for young men to know which other young girls or women had already been secretly "picked off?"
The Tanners, in Changing World, added (p. 225): The shortage of women was so great that some of the men were marrying girls who were very young. Fanny Stenhouse stated:
"That same year, a bill was brought into the Territorial Legislature, providing that boys of fifteen years of age and girls of twelve might legally contract marriage, with the consent of their parents or guardians!" (Tell It All, 1875, p.607).
According to http://www.utlm.org/newsletters/no91.htm, Stenhouse was "at one time had been a firm believer in Mormonism and had even allowed her husband to take another wife. She wrote: "It would be quite impossible, with any regard to propriety, to relate all the horrible results of this disgraceful system.... Marriages have been contracted between the nearest of relatives; and old men tottering on the brink of the grave have been united to little girls scarcely in their teens; while unnatural alliances of every description, which in any other community would be regarded with disgust and abhorrence, are here entered into in the name of God...It is quite a common thing in Utah for a man to marry two or even three sisters.... I know also another man who married a widow with several children; and when one of the girls had grown into her teens he insisted on marrying her also... and to this very day the daughter bears children to her step-father, living as wife in the same house with her mother!" (Tell It All, 1874, pages 468-69)
Per researcher George D. Smith (Source: "Nauvoo Polygamists", George D. Smith, Dialogue: A Journal of Mormon Thought, Spring 1994, p. ix, as found at http://www.utlm.org/newsletters/no91.htm) discovered that of "a list of 153 men who took plural wives in the early years of the Mormon Church. When we examined this list, we noted that two of the young girls were only thirteen years old when they were lured into polygamy. Thirteen girls were only fourteen years old. Twenty-one were fifteen years old, and fifty-three were sixteen years old when they were secretly enticed into this degrading lifestyle."
"I shall not seal the people as I have done. Old Father Alread brought three young girls 12 & 13 years old. I would not seal them to him. They would not be equally yoked together...Many get their endowments who are not worthy and this is the way that devils are made." (Source: Wilford Woodruff, Wilford Woodruff's Journal, 5:58.)
Nice guess; but where's your data proving these women would not get a husband?