1835 D&C declares that the Church believes in monogamy.
Section CI, Doctrine and Covenants (1835), 251, The Joseph Smith Papers website, accessed August 11, 2021
According to the custom of all civilized nations, marriage is regulated by laws and ceremonies: therefore we believe, that all marriages in this church of Christ of Latter Day Saints, should be solemnized in a public meeting, or feast, prepared for that purpose: and that the solemnization should be performed by a presiding high priest, high priest, bishop, elder, or priest, not even prohibiting those persons who are desirous to get married, of being married by other authority
. . . .
Inasmuch as this church of Christ has been reproached with the crime of fornication, and polygamy: we declare that we believe, that one man should have one wife; and one woman, but one husband, except in case of death, when either is at liberty to marry again.