This piece by Murray Rothbard touches on an important point that I think we tend to ignore.
There is, I think, some level of amazement on the parts of politically-correct hoi polloi (I would say "the politically correct hoi polloi, but the word hoi already supplies the "the") as to why there is an innate tendency to take a woman's "virtue" (i.e. adherence to sexual norms) as a relecant factor in judging a rape charge.
Not that, on a poll, most people would admit that a stripper is given less weight than, say, a nun, but most of us have a gut feeling that this ought to be so, even if our rational conscious mind says otherwise. So if logically it would appear that there is no reason to distinguish the rape of a nun from the rape of a hooker, why the instinctual feeling that there is a difference?
Because, the reason why rape is considered so horrible a crime is because of the level of violation that it represents. Sex is an extremely intimate act, and therefore to have it forced upon one is extremely degrading. The degradation is not, whatever feminists tell you, simply about violence. It is about sexual violence. Very few people would consider having one's hand shaken against her will to be in the same league as rape.
In particular, rape represents an attack, not just on the woman's body or her will, but on her chastity, or on her monogamy. Therefore, to some extent there is an instinctual sense that the more casually the woman takes sex, the less seriously a violation forced sex would be; put another way, if sex is like a handshake, then why should forced sex be penalized more than a forced handshake?
This is not to say that it is right to view the rape of a hooker or stripper as a lesser offense. Rather, I am simply explaining where the tendency toward that view comes from.
That is all.