If I may be so bold as to quote an old sitcom: "If you say 'no' when you mean 'yes,' what do you say when you mean 'no'?"
Even if women do mean yes when they say no, any decent man will take the "no" at face value because the consequences of guessing wrong are pretty severe.
Of course one should not take the "no" as "yes". The problem is that very often it means "not yet"--thus the proper reaction is to back off for now and see how things develop. If her answer were truly "no <period>" why doesn't she leave?