The premise is that it is possible to differentiate economic activity from personal activity. Prostitution is economic activity, and there may be some cross substitution effects between prostitution and sex within marriage, but that does not make sex within marriage economic activity. You look at the nature of the activity to determine whether or not it is economic.The argument is clever, I admit, but strictly speaking wrong. It’s essentially a reductio ad absurdum, whose supposedly absurd conclusion is that marital sex is an economic choice. Thing is, the choice to have sex within marriage has numerous economic implications, the effect on prostitution being the least of them. People who have sex within marriage are less likely to buy contraception, less likely to contract STDs, more likely to have children, less likely to purchase alcohol while cruising singles bars, etc. These are assuredly economic effects.
This is not intended as a criticism of Barnett’s position, or even his argumentation. Barnett is making the only argument left to him by a warped interpretation of the Constitution’s commerce clause, which he must take as given in order to persuade the Justices. The current doctrine still interprets the power to regulate interstate commerce as the power to regulate anything that plausibly affects interstate commerce. And the Court has interpreted “commerce” to mean anything “economic” in nature. But as [some blogger I’d link if I could just find the post again] astutely notes, to an economist everything is about economics. So Barnett is forced into the untenable position of arguing for an arbitrary limitation on the meaning of “economic” in order to place any barrier whatsoever in front of an overweening federal government.