But using the "God could have..." rationalization proves nothing. God "could have" worn rhinestones, pleated trousers and a feathered boa, but that's not how Scripture describes Him.
(God was once a man) is something that is not "Doctrine" of the church because the prophet and General authorities of the the church have not come out and said it was "Doctrine"
We are bound by Scripture and the indwelling Holy Spirit for all our beliefs, neither of which says anything about God having ever been a human being.
Unlike Joseph Smith and the Doctrines and Covenants of your church, which say exactly that...
"God himself was once as we are now, and is an exalted man and sits enthroned in yonder heavens!" (Teachings of the Prophet, J. F. Smith p 345). "The Father has a body of flesh and bones as tangible as man's" (Doctrines and Covenants 130:22). "God himself, the Father of us all was once a man like us." (History of the Church, Vol 6, Joseph Smith p 305).
So this bizarre notion that God the Father was once a man is much more than a "personal belief" -- it is part of your "doctrines" and was taught by your "prophet."
Being a prophet has its perks, being God is better