I've heard plenty of good arguments against using heroin. It's extremely (and immediately) addictive. Recovering from that addiction is nearly impossible for most people.
Even if a person safely uses it, and can safely and legally maintain their addiction, heroin is inherently harmful to that person.
Is sex work (prostitution) inherently harmful? I don't think so. Sure, there are plenty of closely related things that are inherently harmful, but sex work can be done without them.
There is the idea that because heroin use often involves second-order effects like shared needles, overdose, etc. that it should be prevented. The reality is that most efforts to prevent heroin use tend to make these second-order effects more prevalent.
Recently there has been a movement to decriminalize drug use, including heroin. The idea is to replace measures that work to prevent heroin use with measures that work to help people overcome heroin addiction, and safely use heroin in the mean time. This way, we fight the second-order effects directly, which is much more effective.
It's a similar story with sex work. Many efforts to decriminalize sex work still have the end goal of getting people to stop engaging in it. This is in contrast to legalization, which would promote sex work while working to minimize the second-order problems that are common to black market sex work.
This is the crux of GP's question: Is there any good reason outside second-order effects to get people to stop doing sex work? I personally am convinced there are none, though my conservative Christian parents disagree.