פירוש על עבודה זרה 50:2

There is a leniency in the laws of seclusion—if a man’s wife is with him, another woman may be isolated with him and his wife. The idea is that the man will have some sense of shame in front of his wife and will not come to have sex with another woman. This, according to this sugya, is true for Israelites but does not apply to non-Jews. They will have sex with another woman even in the presence of their wives.