What types of improper thoughts are prohibited for women?
A woman should not indulge herself with matters which arouse erotic thoughts. From the verse “do not stray after your heart and after your eyes…” – we derive that one may not arouse erotic, promiscuous thoughts.
It is worth adding from Kuntreis Pokeach Ivrim chapter 9, where the Mittler Rebbe strongly admonishes women that they too must protect their eyes and thoughts. They should be careful not to gaze at the beauty of men or engage in idle chatter with men.
Ultimately, such thoughts could also lead to other negative emotions or behavior, and possibly lead to Heaven forbid, outright promiscuity. Hence, one should not engage in such thoughts, even if some forms of these thoughts may not be outright forbidden, and thus spare themselves of the negative spiritual and emotional effects.