Latter-day Saints who go through temple rites covenant and promise before God, angels and witnesses to obey the law of chastity; which is that they will have no sexual intercourse except with their spouse to whom they are legally and lawfully married.
In Illinois during the entire 1840s it was illegal to be married to more than a single spouse. In the United States Territories it was likewise a crime to be married to more than one spouse. Because of open, unlawful cohabitation by members of the LDS church beginning in the early 1850s, Congress enacted harsher and harsher penalties to stop that unlawful conduct. Congress ultimately confiscated LDS church property to compel them to obey the law. Apparently the LDS church valued property more than their religion because they abandoned unlawful polygamous marriage as a tenet of their faith.