With the exception of women who are forced into the industry due to some sort of coercive arrangement, I don't think the concern is that sex workers themselves are being victimized. Certainly, the upper tier MPAs/SPs discussed on this board are doing what they do because they want to, and it proves very lucrative for them. The concern of many feminists centres on the implications of sex work for women in general, as it's said to reinforce the social construction of women as sex objects or even sexual commodities.
Think about what we do on this board. Many of us apply numerical ratings to women, evaluating them based on patriarchal standards of female beauty, how well they serve us sexually, etc. This could be construed as at least somewhat dehumanizing or objectifying when we think about it. And I'm by no means flaming anyone on this board when I write this, as I'm just as guilty of it as anyone! Just trying to step back and see the other side of the argument.