I mean, if it was something special about sex, we wouldn't need to qualify it as sexual (prostitution is not 'sexual prostitution' or even 'sexual slavery').
So what is it that is the problem with sexual objectification? It's not the objectification proper since nobody is lamenting the objectification of plumbers and painters or what not. And it's not like anybody cares about what their feelings are like. Everyone seems to understand that it's a job.
So really, the aspect of sexual objectification people don't like is that they believe no one should be allowed to sell sex. And my question is why? We sure are allowed to sell food, either at the grocery or the restaurant. We're are allowed to sell and rent living arrangements. Although it's uncommon, there's even places charging for access to toilets.
So why is sex so special?