Later on in the play, Stanley exposes Blanche for her seedy misdeeds back in her and Stella's home town. Not only was Blanche run out of town for her multiple encounters with random men, but she also lost her teaching job for having an affair with a 17 year old student. (Scene 7)
Sure, I can see how you would be upset by this. She pulled the wool over your eyes as much as Mitch's!
By exposing Blanche, Stanley strips her of the only thing forming her identity. She is the pure Southern Belle, that is the only way of life she's ever known. If she is no longer considered a pure woman, than who is she really? This, along with Stanley's assault, further Blanche's identity crisis.
In a Streetcar Named Desire, adherence to gender roles (specifically women's) determines a person's worth much like they do in our own society. Women who are dainty and helpless are admired, women who are "used' are considered dirty and unsalvageable, while men are never held to the same standard. Blanche's seedy encounters made her impure in the eyes of Mitch, kick-starting her descent into irreparable mental delusion. However, no one questions Stanley's misdeeds, despite him being the main aggressor of the play. Through his characterization of Blanche and also Stanley, Williams showcases the unfairness of our societal standards for both men and women.