It is mainly because it is an illicit act to have sex with some one for personal game, as it is basically a form of bribery. As the money/act of sex, is an attempt to illicitly have access to something circumventing the typical rule for society.
Typically, if you try to pay for an illicit service, if you lost your money, it is "oh well, you lost out, it is what you get for trying to cheat".
However, the whole act isn't rape. The woman was not forced into the situation, as in examples of Blackmail or by violence. It is akin to meeting some one at a bar, they say they are a neurosurgeon, and they goes "nice, a professional", then sleeps with him, find out he isn't. It wasn't a rape.
Ultimately it is 'meh'
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