You know it could be argued that a no w/ option isn't really a no at all. Regardless of context.
Not really, because circumstances may dictate a change in the mood and therefore the answer, but no is still a no. That's why I say, from the male standpoint, a no must be considered final. It's not the context, it's the consequences.
Look at it this way: If sex is put forth as a possibility during a date, frequently the subject of birth control comes up. The man can say, "I'm fixed," or "I can't have kids" or "I have a condom," or whatever else, and the woman may choose to believe him. However, the immediate consequences, if there are any, belong entirely to the woman (disregarding the semantics of courts and legal paternity cases etc). She is the one facing pregnancy and all the shit that comes with it; therefore, it falls to her to be ultimately responsible for birth control, whether she takes a pill, insists on a condom, or simply says "not tonight."
Conversely, no matter how many times a woman says "Maybe" or "Changed my mind" or "just kidding c'mere ya big lug" after first saying "No" to sex, the consequences legal and otherwise if she later cries rape belong entirely to the male. Therefore, it is his responsibility to protect himself just as it is hers to protect herself. He has to decide if "no means no" and the potential risks are just too great to leave that kind of thing to chance.
So legally, ethically, morally and logically, no has to mean no for the protection of both parties. At the very least, a "no" at an awkward time should cause the man to question the stability of his date for future reference.