Edit to add my opinion so I’m not just replying “I agree” to 90% of comments. I think it should be legal, properly regulated, taxed and viewed as a profession. I haven’t personally engaged in it but I have no moral objection to it. I do hate the common sentiment that it was the individual’s “only option” though.
In my opinion, there are two key could issues with it: societal attitudes and desperation.
Society clearly discriminates sex workers, especially women, as sex work is associated with promiscuity and little limits. Ex-sexual workers are more likely to be harassed and face unwanted sexualized interactions. They have issues finding a long-term partner and may have to hide their past from everyone.
Another issue is that many people choose sexual careers not because they want it, but because it’s one of the few ways to make money quickly. This experience can be heavily traumatic, not only because of the aforementioned societal attitudes, but because of violation of intimacy and losing agency over own body.
So, legalization of prostitution is, at best, harm reduction at this stage.
Important nuance: there absolutely are people who enjoy working in the sex industry. Ideally, healthy incentives and shift in social attitudes would allow them to work in this sphere and others not to. But as things stand, we’re very much not there yet.