Start by thinking that the girl does the job for money, BUT if the girl is in a work arrangement where she can't choose her clients and has to work with several per day (as you wrote), then if she enjoys sex, it will be easier for her to handle the situation, since it's not easy to serve multiple people and there's no option to choose. AND if the focus is on the client, who paid, then I think it's better to make sure he's satisfied. In fact, a man who goes to a place where he realizes the girl can't choose and works with several clients a day, then, from a human perspective, shouldn't expect too much in terms of what he'll receive…
There are girls who don't like oral sex, some will pretend to orgasm even if they don't like it, and others who love it and will benefit from it.
If they prefer to be free from it if they don't like it? It would be better, it would be honest, but there are people who advertise various services and then don't do them, but like to be involved in everything to attract the attention of the potential buyer of the service…
In a perfect world, it's best to offer and do only what a person likes, but depending on the girl, under the conditions she works in, she may not have that option…and she won't be able to admit she doesn't like it.
I once worked for an “agency” in another country, and nobody treated me badly; in fact, they were very good. The drawback was that I wasn't free, in the sense that I could receive someone at any time. And I wasn't obligated to do what I didn't want to do, BUT according to "the agency"—first I had to get the money, then I had to “deal” with what I didn't like. In other words, I acted according to what I am totally against— The Lie. And a detail—I would pay a “fine” if they lost the client they sent me. Therefore I always had to do my best, a real “game of skills”, where I had to "escape" discreetly if I didn't do something… and keep the client satisfied until the end…
Have a good day
Rafaela