What really matters is the safety of the person(s) involved. And yet, there are plenty of occupations where safety is in question.
The question is whether or not we want to be a society that views people's bodies and sexual access to them as a commodity to be bought and sold. I find it baffling that anyone would laud this condition after giving it a few moments of compassionate reflection.
It's all well and good to say that it's against your own morals, but as soon as you start defending that on the basis of the consequences, you're conceding the "but it's just morally wrong!" argument. And in general, we (okay, at least I, as I can believe that you wouldn't necessarily agree here) laud societies that have transitioned from deontological to consequentialist ethics.
And all the more if it's a billion prostitutes in an eternity of misery supported by a malevolent AI!
Alternatively, Alice could consider the possibility that Bob is not, in fact, an idiot.
If you want to get all academic about it, I am coming from a position of virtue ethics, which is why I am focused on the vicious men who purchase sex, and to a lesser extent other people who are explicitly or implicitly defending those men's actions.