DD, I understand how the federal system works.
But it's very easy for the feds to force states to enforce certain certain laws on a national basis. All they have to do is pass a law witholding federal funds from states that don't pass a certain law.
For example, the reason the U.S. has a national drinking age of 21 is that the federal government would withold federal highway funds from states that don't have such a limit (so all states do). Another example was the 1974 "energy crisis" (which was itself created by the federal government) during which the federal government passed a law witholding highway funds from states that did not have a 55 mph speed limit (so all states passed the 55mph speed limit).
If the feds decide to really crack down on prostitution, they can find a way to do it--either through the witholding federal funds method or by attacking the problem through legitimate power over interstate commerce (and prostitution almost always can be shown to involve interstate commerce, especially in an age of travelling escorts, massage girls, etc.).
So don't be complacent...were it not for all the other problems that the US is facing, the feminists would try to get their way through the federal government.