Gen 39 & 40 – How did Joseph know committing adultery was a sin?

Gen 39:9
"There is no one greater in this house than I, and he has withheld nothing from me except you, because you are his wife. How then could I do this great evil, and sin against God?"

This is an enormous declaration of faithfulness by Joseph. How does Joseph know that adultery is sin? And specifically, how does Joseph know this is a sin against Elohim?
Lev. 6:2
“When a person sins and acts unfaithfully against the LORD, and deceives his companion in regard to a deposit or a security entrusted to him, or through robbery, or if he has extorted from his companion,
Num. 32:23
“But if you will not do so, behold, you have sinned against the LORD, and be sure your sin will find you out.
Psalm 51:4
Against Thee, Thee only, I have sinned, And done what is evil in Thy sight, So that Thou art justified when Thou dost speak, And blameless when Thou dost judge.
This is another piece of overwhelming evidence that Torah was known by men before the time of the tablets given on Mt. Sinai. Joseph knows that nothing has been withheld from him in the entire household of Potiphar, except his wife.