I think what it comes down to is this. So far this debate has been framed as a rights issue, that people have the right to sell their bodies. We don't particularly disagree with that, but I think we need to reframe this as a proportional rights issue.
For example, my right to kill ends with everyone else's right to life. My right to purchase sex ends with everyone else's right not to be exploited. There are many things that are against the law and where harm doesn't actually have to happen, but as a society, we've decided are risky behaviours. A great example of that is drunk driving. Far more often than we'd like, things go wrong and people get hurt. So even though most drunk drivers get home safely, we've decided as a society that it's simply too risky. It's unacceptable because of the risk it poses to other people.