That's because it's not true. It's one of many things the anti Mormons put out there to distort our faith.
WHOA! That's gonna be a real disappointment to the mormons I went to church with for years. "As man is, God once was, as God is man may become" was taught over and over again. Boggles the mind that something this ingrained in mormonism is now being denied.
The first chapter of the edition (1992) of the Latter-day Saint teaching manual, Gospel Principles (Church of Jesus Christ Latter-day Saints,1992 ed., p. 9.), quotes from the above passage under the heading What Kind of Being Is God?:
The Prophet Joseph Smith said: "If the veil were rent today, and the great God who holds this world in its orbit, and who upholds all worlds and all things by his power, was to make himself visible — I say, if you were to see him today, you would see him like a man in form" (Teachings of the Prophet Joseph Smith, p. 345). God is a glorified and perfected man, a personage of flesh and bones (see D&C 130:22).
God used to be a man on another planet, Mormon Doctrine, p. 321. Joseph Smith, Times and Seasons, Vol 5, pp. 613-614; Orson Pratt, Journal of Discourses, Vol 2, p. 345, Brigham Young, Journal of Discourses, vol. 7, p. 333.)
Now awaiting usual apologist's response discounting and twisting my sources as lies and my own memory as false...10...9....8....