Originally Posted by Libertine

Personally, I tend towards an ethics mostly based on the second formulation of the Categorical Imperative: that one should always treat others not just as means to an end, but as ends in themselves.

Prostitution, however, is fully compatible with this. It acknowledges the status of prostitutes as rational beings, ends in themselves, and by paying them, it clearly respects the goals that they, as rational beings, set for themselves. The goal in this case being money, of course.