Why? Canaan was just a baby at that time. It doesn’t make the slightest bit of sense.
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the Bible is full of poetic language, ie -not meant to be taken literally, although often a very precise allusion to a very specific concept.
therefore, it is useful to explore the concept of “uncovering nakedness” elsewhere in the OT and see what we find.
https://www.biblegateway.com/passage/?search=Leviticus%2018&version=NKJV
6 ‘None of you shall approach anyone who is near of kin to him, to uncover his nakedness: I am the Lord. 7 The nakedness of your father or the nakedness of your mother you shall not uncover. She is your mother; you shall not uncover her nakedness. 8 The nakedness of your father’s wife you shall not uncover; it is your father’s nakedness. 9 The nakedness of your sister, the daughter of your father, or the daughter of your mother, whether born at home or elsewhere, their nakedness you shall not uncover. 10 The nakedness of your son’s daughter or your daughter’s daughter, their nakedness you shall not uncover; for theirs is your own nakedness. 11 The nakedness of your father’s wife’s daughter, begotten by your father—she is your sister—you shall not uncover her nakedness. 12 You shall not uncover the nakedness of your father’s sister; she is near of kin to your father. 13 You shall not uncover the nakedness of your mother’s sister, for she is near of kin to your mother. 14 You shall not uncover the nakedness of your father’s brother. You shall not approach his wife; she is your aunt. 15 You shall not uncover the nakedness of your daughter-in-law—she is your son’s wife—you shall not uncover her nakedness. 16 You shall not uncover the nakedness of your brother’s wife; it is your brother’s nakedness. 17 You shall not uncover the nakedness of a woman and her daughter, nor shall you take her son’s daughter or her daughter’s daughter, to uncover her nakedness. They are near of kin to her. It is wickedness. 18 Nor shall you take a woman as a rival to her sister, to uncover her nakedness while the other is alive.

24 ‘Do not defile yourselves with any of these things; for by all these the nations are defiled, which I am casting out before you. 25 For the land is defiled; therefore I visit[c] the punishment of its iniquity upon it, and the land vomits out its inhabitants. 26 You shall therefore [d]keep My statutes and My judgments, and shall not commit any of these abominations, either any of your own nation or any stranger who dwells among you 27 (for all these abominations the men of the land have done, who were before you, and thus the land is defiled), 28 lest the land vomit you out also when you defile it, as it vomited out the nations that were before you. 29 For whoever commits any of these abominations, the persons who commit them shall be [e]cut off from among their people.
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other translations discard the poetic language of the KJV in Leviticus, and just baldly state that this is a list of people you’re not supposed to be having sex with. on pain of death. which necessarily obscures the actual meaning of that post Flood interlude.
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and which also raises the point that Noah failed to kill Ham for his sin of incest.
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now we turn to the specific question of “why Canaan”?
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https://www.biblegateway.com/passage/?search=Genesis+9%3A18-28&version=NKJV
you will note that it is stated repeatedly that Ham is the father of Canaan. what is NOT said?
– it does not say that Canaan has yet been born
– it does not mention any of Canaan’s brothers, which is curious as Canaan is the LAST of Ham’s sons to be listed in Gen 10. obviously implying that Canaan is the youngest. also implying that something was done to prevent Ham fathering any more children.
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i would not say that i have demonstrated a “proof” that Naamah was Canaan’s mother. but it doesn’t look good.

https://drmsh.com/naked-bible-podcast-episode-159-noahs-nakedness-the-sin-of-ham-and-the-curse-of-canaan/

Nutshell: Heiser’s take is that Ham raped his mother as a power play over a drunk Noah. Canaan was the offspring of that interaction. Dealt with in the Torah later, boinking someone besides your wife will make problems. In this case, Ham opened the door or partnered with the fallen to reseed creation with the seeds of the serpent/Nephilim/giants,

A different perspective, a similar outcome from your rundown at the top. Stuff that should be taught in church.