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So what's up with that? It sounds almost like a relationship. If what men are really seeking is variety, why the repeat customers?
Probably a case of different strokes for different folks. It may be a bit like going to the same barber for a haircut 100 times, if it is just a comfortable ritual. He may get to like her and enjoy her company even if the sex is not the only thing. He can also talk to her about all kinds of personal revelations and sexual topics that maybe he can't discuss elsewhere, so it may be therapeutic and relaxing to have this secret, alternate life.
Also note that in the US (was this study in the US?) prostitution is not legal (except Nevada brothels) and so a man is at some legal risk on the first encounter with a new paid partner. Once he has been with her once, he is home free, so to speak as far as her being a police decoy, and if she delivers what he is looking for from the get go, then returning is easy and risk free. If he is married, this is particularly important.
Paul Theroux's book My Secret History is an interesting example of a behind-the-scenes look at a man's sex life.