Why do Mormons believe you are damned if you don't practice plural marriage?

"Now if any of you will deny the plurality of wives, and continue to do so, I promise that you will be damned," (Journal of Discourses, vol. 3, p. 266). Also, "The only men who become Gods, even the Sons of God, are those who enter into polygamy," (Journal of Discourses, vol. 11, p. 269).