(TLDR: Because a significant % of prostitutes are forced/coerced into it, meaning when you sleep with a prostitute there’s a decent % chance you’re raping someone)
When I did debating, most arguments I would hear against prostitution were about the innate wrongness of it. Prostitution was bad because it’s morally wrong to trade sex. Prostitution was bad because it’s exploitative of sex workers as they have to engage in it to eat. Prostitution is bad because you can’t meaningfully consent to sex if money is involved etc… I never found these arguments persuasive. Logically, if you can consent to working other dangerous jobs such as coal mining or the military you can consent to the dangers/hardships of life as a prostitute. In terms of exploitation I think people should be allowed choose for themselves if they would rather be exploited or unemployed/employed in a shitty job doing manual labour. Still, my intuition was that there was still something wrong other people weren’t hitting on. In retrospect the reason why it’s wrong, at least from the buy side, is obvious. A decent percentage of prostitutes in first world countries are controlled by pimps, coerced, trafficked and generally can’t really say no. Hence purchasing sex from a prostitute means running a high risk of raping someone. Hence it’s wrong to buy sex from a prostitute.