They know what is respectful and disrespectful because they can put on the act of being respectful during the early stages of dating. In order to put on an act of being respectful, you have to know what being respectful is, even if you're faking it. The knowledge is there in their brain and many women use this act to rope a man in during the early stages of dating, only to change for the worse once they've been seeing each other for a while and the man is emotionally invested.