So, as I understand it, Joseph had the sealing power given him by God, a priesthood that cannot be passed down.1This enabled him to save those who were sealed to him2, by the power that he held (which type of sealing cannot be passed down from man to man). Thus, it seems to me3, especially in light of what I have read here, that Joseph was sealed to women, not as a sexual partner but as a power to raise up seed unto the Lord, meaning souls who could be saved through being sealed to Joseph. It was no more sexual than when he had sons sealed to him. Thus, Joseph could honestly say he had only one wife and was not practicing polygamy because he understood others to mean having sexual relations with other women, when that was not the case. But through the years, the doctrine was misunderstood and changed.
This would explain why there are no physical descendents of Joseph except through Emma - a woman sealed to Joseph might consider him the father all of her children, even though her “legal and lawful” husband provided the actual seed. She might even confide in her child/ren that Joseph was their father.
And we cannot really know how many women were sealed to Joseph in his lifetime, with his presence and consent. By the time plural marriages became well-known, there was a stake in “proving” that Joseph had practiced plural marriage. Those who testified that they had been Joseph's wives, may have been sealed to him (in a nonsexual relationship), or they may have been “lying for the Lord”.
There were also people sealed to him as his children. I have a great-grandfather back some generations who was sealed to Joseph as his son (but the sealing wasn't done until the pioneers were in Utah - unless it was redone then). But Joseph didn't treat them as children, I expect. For example, they didn't have to worry about a whipping if they did wrong (the common punishment of children in those days).
Anyway, this is my latest understanding of the idea of Joseph and plural marriage.
1 JST 14 27. And thus, having been approved of God, he [Melchizedek] was ordained an high priest after the order of the covenant which God made with Enoch,
28. It being after the order of the Son of God; which order came, not by man, nor the will of man; neither by father nor mother; neither by beginning of days nor end of years; but of God;
29. And it was delivered unto men by the calling of his own voice, according to his own will, unto us many as believed on his name.
2 I cannot find the exact words I seek, but I understand it to be what Joseph preached. Here is an example of something he said that may refer to that: I will walk through the gate of heaven and Claim what I seal & those that follow me & my Council - Wilford Woodruff Diary 10 March 1844 (Sunday). At Temple. (Words of Joseph Smith)
3 “Joseph's practices were carefully guarded, hidden from public view, and so discrete that still today there are those who think he never had plural wives. If this were something for public display and advocacy, then Joseph would have done so. He did not. To the contrary, he also delighted in the chastity of women and condemned adultery and fornication.
“It is my view that the question of taking plural wives arises with Joseph Smith, and was through a revelation to him when he inquired about the topic. He treated it as a limited, carefully curtailed, private matter. His implementation of the practice was limited to sealing his own plural wives, and one other man to two wives.
“With Brigham Young, however, taking more women became not only public, but it also became a topic used to prove his own v[i]rility.”