There is literally nothing wrong with sex as transaction. It probably is unironically as old as humanity itself. And, controversial opinion, it is probably a lot more common today than we think, whether in relationships, or outside them, but I do, it is worth mentioning, have a cynic bias.
Why do I care? Let me put it this way. I am autistically pedantic. But also, I believe in earnest the whole moralizing prostitution thing is really a mask for gender narratives. "Women are helpless muffins". I find this appalling for two reasons: the first, because it is deeply chauvinist when you turn on your brain, the second, because some men might be naive enough to fall for it and get taken advantage of by women as a result.
No matter how the moralizers try to frame it as being about whether sex can be "work" or not, in reality, the activity itself, and its semantics, are irrelevant in terms of human suffering.
The actual causes of suffering:
1. Pimps (who I consider on the same level as slavers, and would treat the same as the good ol' John Brown did if I could)
2. Cops (being branded a criminal isn't very nice)
3. Violent Johns (but independent prostitutes nowadays have online platforms where they gossip about and warn of clients to other prostitutes)
4. STDs (admittedly still a risk even with medical advancement)
That's it.
Any discussion that does not revolve around these is useless moralizing.