Well he was not impotent and it seems to me if there are no children from his other "unions" there really is no slam dunk.
These were furtive, hidden relationships, not relationships in which regular sex was occurring. It is not uncommon at all for these sort of relationships NOT to result in conception.
When the RLDS church was formed, partly in protest of polygamy, its leaders originally denied Joseph Smith ever practiced polygamy and pinned the practice on BY. In response, the LDS church gathered affidavits and interviewed Joseph Smith" still living wives. Those responses left little doubt that not only did Joseph Smith marry these women, but had sexual relations with many. Of course they did not use the words "intercourse", but instead used words such as "in very deed".
Moreover, Brigham Young and other church members who practiced polygamy had been taught what polygamy meant from Joseph himself. They all understood it was a real marriage, that included sexual intimacy and hopefully children.
I can't think of one logical reason to think that, particularly with the "revelation" that Joseph Smith used to justify polygamy which expressed stated it was for the purpose of raising seeds, Joseph Smith did not have sexual relationships in his polygamous marriages.
There are only a few marriages in which it is speculated sexual relations may not have occurred, and that is his marriages with much-older women, or, perhaps, the younger Helen Mar. The older women were likely post-menopausal, and Helen Mar was so young at their marriage she may not have been sexually mature.
I also am under the impression that the DNA research has not been completed. But, as I said, even if he did not impregnate any of these women, would that be a surprise? He wasn't living with them. He was constantly trying to find ways to meet with them without Emma's knowledge. On top of that, he was juggling a LOT of wives. I would imagine that sexual contact with any one wife in particular, other than Emma with whom he lived, was likely very infrequent.
This page discusses these sort of evidences:
http://www.i4m.com/think/history/joseph_smith_sex.htm
Note in particular these statements:
Compton writes:
"Because of claims by Reorganized Latter-day Saints that Joseph was not really married polygamously in the full (i.e., sexual) sense of the term, Utah Mormons (including Joseph's wives) affirmed repeatedly that Joseph had physical sexual relations with his plural wives-despite the Victorian conventions in nineteenth-century American religion which otherwise would have prevented mention of sexual relations in marriage."
- Faithful Mormon Melissa Lott (Smith Willes) testified that she had been Joseph's wife "in very deed." (Affidavit of Melissa Willes, 3 Aug. 1893, Temple Lot case, 98, 105; Foster, Religion and Sexuality, 156.)
- In a court affidavit, faithful Mormon Joseph Noble wrote that Joseph told him he had spent the night with Louisa Beaman. (Temple Lot Case, 427)
- Emily D. Partridge (Smith Young) said she "roomed" with Joseph the night following her marriage to him and said that she had "carnal intercourse" with him. (Temple Lot case (complete transcript), 364, 367, 384; see Foster, Religion and Sexuality, 15.)
In total, 13 faithful latter-day saint women who were married to Joseph Smith swore court affidavits that they had sexual relations with him.
- Joseph Smith's personal secretary records that on May 22nd, 1843, Smith's first wife Emma found Joseph and Eliza Partridge secluded in an upstairs bedroom at the Smith home. Emma was devastated.
William Clayton's journal entry for 23 May (see Smith, 105-106)
- Smith's secretary William Clayton also recorded a visit to young Almera Johnson on May 16, 1843: "Prest. Joseph and I went to B[enjamin] F. Johnsons to sleep." Johnson himself later noted that on this visit Smith stayed with Almera "as man and wife" and "occupied the same room and bed with my sister, that the previous month he had occupied with the daughter of the late Bishop Partridge as his wife." Almera Johnson also confirmed her secret marriage to Joseph Smith: "I lived with the prophet Joseph as his wife and he visited me at the home of my brother Benjamin F." (Zimmerman, I Knew the Prophets, 44. See also "The Origin of Plural Marriage, Joseph F. Smith, Jr., Deseret News Press, page 70-71.)
- Faithful Mormon and Stake President Angus Cannon told Joseph Smith's son: "Brother Heber C. Kimball, I am informed, asked [Eliza R. Snow] the question if she was not a virgin although married to Joseph Smith and afterwards to Brigham Young, when she replied in a private gathering, "I thought you knew Joseph Smith better than that."" (Stake President Angus M. Cannon, statement of interview with Joseph III, 23, LDS archives.)