Why do women flaunt their vulnerability around men and then wonder why they got taken advantage of?

Why do women go over to a man's home when emotionally vulnerable (recent breakup, death in the family, weight gain, etc) and expect the man not to take advantage of her weaken state? Men have to play aggressive offense 24/7 since women are constantly playing defense. When we see an opportunity to pounce we take it without hesitation. So why do women flaunt their vulnerability around men and get upset the following morning when she realizes that she let her guard down?