Seems odd, and unfortunate, to me that sex workers would develop contempt for their clients.
A few possible reasons:
Some clients, especially those from more patriarchical cultures, have a low opinion of "whores". After having paid, they feel they can do with them as they please.
Societal opinion says: only a loser will pay for sex, because he has no other options. So a sex worker has to earn a living by having sex with "losers" who are often old enough to be her father or grandfather. And she has to pretend to like it.
The money. Sex work can be a kind of "golden cage": the hourly wage is much more than can be earned in most "normal" jobs. So even if you'd rather stop, actually doing so can be very difficult because of the substantial pay cut you'll have to endure. Many sex workers keep working reluctantly because they got used to the money.