Suppose an evil twin tricks his brother's wife into having sex with him (by pretending to be his brother, her husband). This is presumably rape. The deception seems, in this case, to nullify the target's consent (much as hypnosis, brainwashing, or immaturity might). We might capture this by suggesting that the woman only consented to have sex with her husband, not with his evil twin. The action she actually ended up with is not the one she consented to.
However, it isn't clear that misinformation always nullifies consent. Suppose a Tom Cruise impersonator picks up a girl at the bar, who would not have gone home with him if she knew his true identity. There's something morally dubious about this, but intuitively it doesn't seem to reach the bar for rape. But why not? Can't she say that she only consented to have sex with TC, not with his lookalike?
An obvious difference between the two cases is that only the former involves a pre-existing relationship/personal connection. But why does this make such a difference? One possibility is that we clearly have a weighty and legitimate interest in discriminating between potential sexual partners on the basis of existing personal connections. Impersonation of a personal acquaintance is thus an especially egregious form of fraudulent exploitation. But one's interest in having sex with a stranger who truly satisfies certain conditions may not call for quite the same degree of respect.
Does that sound right? It seems a bit harsh to delegitimize certain preferences like that. But I'm not sure how else to explain the moral difference between the two cases. (Any suggestions?) Also, does this proposal yield the intuitively right results in other cases, or does it need refinement?