A commenter named DCLaw1 over at Glenn Greenwald’s place phrases the question that I didn’t even know was on my mind until I read it:

I have always found it very curious that one of the following, but not the other, is illegal:

(a) Two people have sex, one of them gets paid for it;

(b) Two (or more) people have sex, all of them get paid for it, and it is videotaped and sold to third parties as a commodity.

I have yet to hear a convincing argument why this difference makes any actual sense.