What was his defense? We know, ultimately, that he fled. But we also know that he saw—from the very beginning—what was happening for what it really was. In verse 9, he said:
First of all, young as he was. . . I do not know how old he was. Maybe by this time several years had passed and he was in his early twenties, mid-twenties? I do not know. But, whatever it was, he was in the bloom of his youth and very appealing to this lady. But he saw that it was sin. He knew that. He saw that what was happening was not love. It was lust. And he understood that sex is good. That it is sanctified by God, but only if it is used as God ordained it—within marriage.
He also saw, in verses 8 and 9, that this was going to be destructive to his master, and besides that, it was going to ruin his own employment as well. He says, "Look my master, your husband, doesn't have a concern about what's going on because he believes that it's in good hands. Am I going to be disloyal to that?"
He obviously understood that there would have been a sin against Potiphar, but to Joseph, the far worse thing was his sin against God.
