It is a requirement that force or the threat of force must be present to violate a rights. Bank robberies and muggings are sometimes not actively resisted because of the threat of force. Rape is accomplished by force or the threat of force, usually a much longer and drawn out sequence of pushing around, threatening, disrobing, striking . ... etc than the "surprise sex" of this contrived scenario. Sally is not presented with any threats or employment of force and gives every sign of consent to what happens in the darkness up until the moment that penis is in vagina, and then nothing changes after that moment. Chris doesn't need to use force or even a threat of force. Why does Chris not need to use force or the threat of force? Because Sally is cooperating and participating, and actively consenting in the make-out session up until the moment of penetration. After the moment of penetration nothing changes except in the secret recesses of Sally's thoughts. The consent that was present the moment before penetration is by every outward sign still present after the moment of penetration.
And no, that she said she didn't want to have sex hours ago does not mean she did not change her mind. To put the point positively, it is always in Sally's power to decide to have sex. That decision is communicated by actions and words, but in this case actions speak louder than words because Sally decided not to use any words. All her actions said "yes".