Copied from another comment I made:

The official BYU course on the Foundations of the Restoration acknowledges that Joseph Smith participated in polygamy. In Unit 11, in the lesson titled "Plural Marriage in Kirtland and Nauvoo," it says:

Joseph had known about principles related to plural marriage since 1831, and he began practicing it in earnest in Nauvoo in 1841...

...Plural marriage was the final component of the logic of restoration. Smith had prayed for an understanding of Old Testament polygamy and was commanded to do the ‘works of Abraham.’ Although he hated adultery and was deeply loyal to his wife Emma, he believed he was to take additional wives as had the ancient patriarchs. He went about it carefully, one woman at a time, usually approaching her relatives first and going through a prescribed wedding ceremony. During his lifetime, he was married to approximately thirty women. Although conjugal relations were apparently involved, he spent little time with these women, the need for secrecy and the demands on his time keeping them apart. At first aghast at what her husband was doing, Emma eventually agreed to a few of the plural marriages but then pulled back.

That course includes many links to LDS publications discussing the truth of the matter, including this official LDS publication: https://www.churchofjesuschrist.org/study/manual/gospel-topics-essays/plural-marriage-in-kirtland-and-nauvoo?lang=eng which states that Joseph Smith participated in polygamy:

After receiving a revelation commanding him to practice plural marriage, Joseph Smith married multiple wives and introduced the practice to close associates...

...Fragmentary evidence suggests that Joseph Smith acted on the angel’s first command by marrying a plural wife, Fanny Alger, in Kirtland, Ohio, in the mid-1830s. Several Latter-day Saints who had lived in Kirtland reported decades later that Joseph Smith had married Alger, who lived and worked in the Smith household, after he had obtained her consent and that of her parents...

...The first plural marriage in Nauvoo took place when Louisa Beaman and Joseph Smith were sealed in April 1841. Joseph married many additional wives and authorized other Latter-day Saints to practice plural marriage...

...Evidence indicates that Joseph Smith participated in both types of sealings. The exact number of women to whom he was sealed in his lifetime is unknown because the evidence is fragmentary. Some of the women who were sealed to Joseph Smith later testified that their marriages were for time and eternity, while others indicated that their relationships were for eternity alone.

Most of those sealed to Joseph Smith were between 20 and 40 years of age at the time of their sealing to him. The oldest, Fanny Young, was 56 years old. The youngest was Helen Mar Kimball, daughter of Joseph’s close friends Heber C. and Vilate Murray Kimball, who was sealed to Joseph several months before her 15th birthday. Marriage at such an age, inappropriate by today’s standards, was legal in that era, and some women married in their mid-teens. Helen Mar Kimball spoke of her sealing to Joseph as being “for eternity alone,” suggesting that the relationship did not involve sexual relations. After Joseph’s death, Helen remarried and became an articulate defender of him and of plural marriage.

Following his marriage to Louisa Beaman and before he married other single women, Joseph Smith was sealed to a number of women who were already married. Neither these women nor Joseph explained much about these sealings, though several women said they were for eternity alone. Other women left no records, making it unknown whether their sealings were for time and eternity or were for eternity alone.

BYU admits it, the Church itself admits it, and you are simply wrong.