For instance, videotaping the action and then blackmailing people with it. Or otherwise causing trouble outside of the direct transaction. And of course, the other side of it, "services" to prevent that from happening outside of the legal system.
Also I feel like I'm pointing out something that's both disgusting and obvious, but I feel like it needs pointing out. More than a bit of sex is painful. For both sexes, but especially for women. There are reasons for that, which boil down to there being a limit to how much friction skin can take, no matter the amount of lubrication (which obviously doesn't tend to be 100% in order, because while you are "the best ever", so is everyone else). This means that sex work other than perhaps really high class escorts is not voluntary, and even then ... It also means that sex workers quickly gravitate to other options than direct intercourse, of which there are thankfully many.
But making matters worse, this is not the sort of activity that is executed in the calmest and politest of settings, and by it's nature does not promote reasoned discussion. Or to put it more to the point: you're going to get hit hard, accidentally, and you're going to get beaten up, non-accidentally, and you're going to have to beat up people. It's just going to happen. Also, the whole point is to let self-control go for at least a while, which can lead to an otherwise reasonable party being unwilling to stop, say, twisting an arm until a certain event that may take a minute or two to occur.
Thirdly, like anything that involves a lot of endorphins, you can get addicted to it. It can get bad: you can literally get addicted to the pain of overdoing it (as in people who cut themselves open during sex, sometimes without telling the "other side of the transaction" they're going to do this). Even in much milder cases, it's still going to cause abuse and violence.