To allow man to be tempted so that he can choose his course and be rewarded for his proper choices, Hashem put another component in man: imagination. Imagination is the creative ability to form a mental picture and feel it as vividly as if it were real.
Armed with an imagination, man can create fanciful worlds at his will and actually believe them. If man wishes to turn to evil, he can create rationales to make these ways sound noble and proper – and fool himself at least. If he wishes he can do what is right, or if he wishes he can turn to wickedness. Even his brilliant intellect won’t prevent him. He is capable of creating entire worldviews that explain how the behavior he desires is righteous, correct, and appropriate. Man has free will.
The answer to this Rashi has two levels. First, we see the power of rationalizing. Even a fully mature, pious woman who grew up in the best of homes can be convinced, on some level, that illicit relations are permitted. The yetzer hara will use her imagination and create clever and creative ways to explain that black is white, in is out, and arayos is permitted. As ridiculous as it sounds, that is the power given to the yetzer hara.
The second idea is that even the woman who seems to be off the derech and wouldn’t need an excuse really does. No human can ever do something that is wrong. Because of the greatness of her soul and the truth she knows deep down inside, she understands that for a married woman to go to another man is forbidden. The only way she can perpetrate this act is if she has a rational way of explaining how in fact it is permitted. The human is incapable of doing something wrong. The only way he can do something wrong is by making it right.