Defense of Marriage Act - Wikipedia
The Holy Bible: King James Version
Whether Joseph's "marriage" to Fanny Alger occurred in 1833 or 1835, it was illegal both under the laws of the land and under any theory of divine authority. Plural marriages are rooted in the notion of "sealing" for time and eternity. It is claimed that the "sealing power" was restored 3 April 1836 when Elijah appeared to Joseph and committed the sealing keys into his hands. (, The Joseph Smith Papers.) Until that time no one on earth had authority to "seal" Joseph and Fanny. As a result, his marriage to her was a nullity from the beginning both in time and eternity, and any sexual relationship he had with her was adulterous.
A look at courtship, love, marriage and divorce in Viking Scandinavia
Fanny Alger was a teen-aged servant in the Smith's home. Joseph and Emma had "adopted" Fanny when she was about 16 years old (1833). She is believed to be either Joseph Smith's first polygamous "wife" or simply a sexual encounter. (The Church's essay, "Plural Marriage in Kirtland and Nauvoo," says it was a marriage, whereas Lawrence Foster said, "…contemporary evidence strongly suggests that Smith sustained sexual relations with Fanny Alger, it does not indicate that this was viewed either by Smith himself or by his associates at the time as a 'marriage.'" Dialogue Vol. 33 No. 1 pp. 184-86.) Critics believe he had an affair with her, was found out, and then introduced the concept of plural marriage in order to justify and continue his affair with her and then other women.