Genuinely curious -- is there a reason not to exploit someone who voluntarily enters into that relationship? I always assumed 'exploitation' meant forcing people to do things they didn't want to do.
Well, it's a moral question and therefore there is no universal answer, everyone decides it for themselves. And given the fact that when presented with a choice between starving under a bridge or working a back-breaking work for 12 hours per day, most of the people historically have voluntarily chosen to labour, one could argue that indeed it is entirely ethical.