Writer's Block: Eliot Spitzer's Resignation

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I have to admit I'm wondering at which point in what he did - and prostitution generally - becomes a major scandal. After all, it's sex between two consenting adults.

Is it because:

a. He paid for it?
b. He had sex with someone he wasn't married to?
c. Prostitution is illegal Stateside?
d. He got caught?

Which bit is the worst or most scandalous out of the whole thing?