It's not about "having sex". It's about selling human bodies for money, to be used for the selfish sexual gratification of others. And most cultures throughout history have understood that this is a damaging practice for them to allow. Yes, it has always happened. But it was not a generally accepted practice, because people knew it was not a healthy way for humans to behave. And the fact that you don;t see the difference is evidence of the damage it does to a cultural mind.
I guess some of it might depend on how "transactional" these relationships might be. I've heard some people argue that marriage itself is a transactional relationship, not necessarily based in love or romance. Nature gives humans the desire to have sex and procreate, and human societies have adapted it into their cultures. Traditionally, men would provide protection and provisioning, whereas the women would carry the children and care for them until they're old enough to take care of themselves.
Prostitution seems more a twisted variation of the same basic theme. Men get their sexual desires fulfilled, and women get provisioning in return. In small numbers, it's something that cultures can absorb and tolerate to a degree, but you probably couldn't build an entire society on it.
The problem is that those lines are being drawn based on the wrong premise. They should be drawn based on the ethical ideals involved, not the visual content. But that would force us to confront the real damage done by greed run amok in our culture, and we aren't (those in power aren't) willing to look at that.
Yes, I can agree with this, but ultimately, it does come to society's views on sex and the process of "having sex," even if, as you say, the topic isn't really about that. One might argue that sex can be a beautiful and wonderful thing between two or more people who love each other deeply, but to make it transactional and tainted by money somehow spoils it and makes it more degrading. But I would ask, what's the core problem here? Sex or money?
If the issue is sex, then, in the context of teaching future generations, society should teach them to be respectful of other people's rights and a more mature, enlightened view on sex and relationships. It seems that society went awfully fast from a more puritanical view of "sex is dirty" to the sexual revolution and a kind of "anything goes" popular culture which is practically everywhere these days. It goes up and down the spectrum. No wonder so many people are confused.
If the issue is money and capitalism, I think most people here know my views on capitalism - and I think you and I see eye-to-eye on that topic, for the most part. The sex industry is extremely capitalistic and often associated with organized crime. So, I haven't forgotten that part, but there may be sex workers who are independent and own the means of production, so to speak. If the worker has control over the means of production, then the worker has choice, freedom, and agency. If the capitalists control the means of production, then the worker is exploited.