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Webcat says: "No" and "Please Stop" were always taken seriously.
I think that's the only thing we can do when, as Anonymous pointed out, there are ambiguities in sexual encounters.
What's the worst that could happen if you stop when the woman says stop? Maybe she meant "stop, that's hurting, so let's change positions." So you do.
But if you don't stop when she says stop...
Also, Anonymous, for what it's worth, the law does recognize that rape isn't a black and white issue. Hence the different classes of rape. First degree or second degree, rape or sexual offense, statutory.
http://www.rapecrisisonline.com/law.htm
It's a recognition, although there's certainly no total clarity.