They looked the other way so they would not see him naked - Genesis 9:23
Certain offices command respect:
Noah's two eldest sons respected their father positon even when he drank too much wine and passed out
Their brother, Ham, on the other hand sinned aginst Noah and God by staring at his father - Genesis 9:22
Shem and Japheth went out of their way to cover their father - walking backward into the tent and covering him
They did everything to protect their father's honor and enjoyed a blessing which their brother was excluded from