It's exploitation because Alice gains more than Bob in the exchange due to her position of power. And it's exploitation whether Bob likes the arrangement or not, unless he specifically wants to be exploited as in the case of a fetish.
I already said that this arrangement takes place in your hypothetical society, so there is no position of power. Bob can work elsewhere, but he chose to work for Alice instead. Your ideology cannot handle two people making a voluntary agreement without labeling it as exploitative, so I think we're not going to be able to have any further rational discussion.