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.
Edit: you say recovering from heroin addiction and second-order effects are all negative as if sexwork has no impact on, for example, human relationships, and while I like some of the thought that went into your post, it still reads as a bit absurd.
not really. 95+% of the population will experience some sexual activity at some point in their life. It is argued as a basic biological need in some psychologies, and a basic pscyological/social need for the rest that may not want to compare it to food/water/shelter. 95% of the population will never do heroin. I argue over half will never even see heroin tools.
That's what makes the controlling of sex more contentious than a very addictive drug. It's a normal (but private) activity that the vast majority of society participates in. It's legal to do it for free, it's legal to pay for content that has people having sex in it. What makes the barrier of paying to do it yourself different?
You've completely removed all the main "negatives" of prostitution, saying those aren't "real arguments."
While it's possible that heroin injection and prostitution may be done in a safe and clean way, the reality is extremely far from that in both cases.