I think women have been conditioned to believe that they can "lead on" (flirt with?) men beyond a point where it's fair to refuse to have sex. But now we're realizing the lie in that. It's never too late to say no. Your partner might get frustrated or mad, but morally, men and women both have the right to change their mind about having sex at any time. And morally, no one should even want to have sex with someone who doesn't want to in return.