Surely, "seeing the nakedness of someone" is not always the same as having sexual relations with someone. It can mean that, but it does not have to mean that. In the text of Genesis itself, it is clear that Noah lay uncovered in his tent. He was naked in his tent. This makes it extremely unlikely that Ham had sexual relations with Noah's wife. The subject of nakedness is Noah himself. Therefore, the usual interpretation of Noah being there naked, and Ham happening upon him naked makes the most sense. The contrast of Ham's behavior with Shem and Japheth's behavior makes it clear that Ham's behavior was not honoring to Noah, whereas Shem and Japheth were honoring. It is not too much of a stretch, then, to suppose that when Ham talked about it with his brothers, it was in a "Hey, did you see our dumb dad? He's lying there naked in the tent! What a sot!" kind of way. It was not a sin to happen upon Noah naked. It was what he did do and what he did not do that made him at fault. Remember also that sin is covenantal, especially honoring one's parents. Just as Ham the
son of Noah did not honor Noah, the curse came upon Ham's
son. The punishment fits the crime. See
here for my sermon on the passage.