Or HaChaim on Genesis 39:9/אור החיים על בראשית ל״ט:ט׳

1 באשר את אשתו, "inasmuch as you are his wife." Joseph meant that Potiphar had not placed his wife out of bounds to Joseph in so many words; the matter was understood because of her status.

2ואיך אעשה הרעה הגדולה, "how could I possibly commit such a great evil?" By doing so I would lose everything I have achieved.

3Furthermore, I would create an unbridegable gap to the holy roots of my soul seeing that adultery is forbidden to Gentiles as well as to Jews. This is what Joseph meant when he said "I would sin against G'd."

4Furthermore, by mentioning that he would commit a sin vis-a-vis G'd Joseph already countered Mrs Potiphar's argument that her husband did not need to know about their liaison and that therefore Joseph did not stand to lose anything. Joseph attributed his present status to the help of G'd. He would certainly forfeit G'd's help if he became guilty of adultery.