Criticism
Critics charge that Joseph Smith’s early plural marriage(s) cannot have been “real” marriages, since the doctrine of “eternal marriage” (i.e., marriages which last beyond the grave) was not introduced until 1841.
Response
Gary J. Bergera, an advocate of the “affair” theory, wrote:
I do not believe that Fanny Alger, whom Compton counts as Smith’s first plural wife, satisfies the criteria to be considered a “wife.” Briefly, the sources for such a “marriage” are all retrospective and presented from a point of view favoring plural marriage, rather than, say, an extramarital liaison…Smith’s doctrine of eternal marriage was not formulated until after 1839–40.[1]
There are several problems with this analysis. While it is true that sources on Fanny are all retrospective, the same is true of many early plural marriages. Fanny’s marriage has more evidence than some. Bergera says that all the sources about Fanny’s marriage come “from a point of view favoring plural marriage,” but this claim is clearly false.
Hostile accounts which report a marriage or sealing
For example, Fanny’s marriage was mentioned by Ann Eliza Webb Young, a later wife of Brigham Young’s who divorced him, published an anti-Mormon book, and spent much of her time giving anti-Mormon lectures. Fanny stayed with Ann Eliza’s family after leaving Joseph and Emma’s house, and both Ann Eliza and her father Chauncey Webb[2] refer to Joseph’s relationship to Fanny as a “sealing.”[3] Eliza also noted that the Alger family “considered it the highest honor to have their daughter adopted into the prophet’s family, and her mother has always claimed that she [Fanny] was sealed to Joseph at that time.”[4] This would be a strange attitude to take if their relationship was a mere affair. And, the hostile Webbs had no reason to invent a “sealing” idea if they could have made Fanny into a mere case of adultery.
The keys?
Some have wondered how the first plural marriages (such as the Alger marriage) could have occurred before the 1836 restoration of the sealing keys in the Kirtland temple (see D&C 110

. Again, this confusion occurs because we tend to conflate several ideas. They were not all initially wrapped together in one doctrine:
plural marriage - the idea that one could be married (in mortality) to more than one woman: being taught by 1831.
eternal marriage - the idea that a man and spouse could be sealed and remain together beyond the grave: being taught by 1835.
“celestial” marriage - the combination of the above two ideas, in which all marriages—plural and monogamous—could last beyond the grave via the sealing powers: implemented by 1840-41.
Thus, the marriage to Fanny would have occurred under the understanding #1 above. The concept of sealing beyond the grave came later.
Conclusion
The Fanny Alger marriage illustrates many of the difficulties which the historian encounters in polygamy. There is little information available, much of it is second hand, and virtually all of it was recorded “after the fact.” Even the dates are unclear, and subject to debate.
It seems clear, however, that Joseph, Fanny’s family, Levi Hancock, and even hostile witnesses saw their relationship as a marriage, albeit an unorthodox one. The witness of Chauncey Webb and Ann Eliza Webb Young make it untenable to claim that only a later Mormon whitewash turned an affair into a marriage
en.fairmormon.org/Polygamy_book/Introduction_of_eternal_marriage
I think that this explains it quite well with fanny.