I like to think it isn't arbitrary, since it is derived from deep personal reflections, as well as scientific findings in domains such as psychology and sociology, which it's not worth detailing now.
The same can be said for literally any moral judgment ever. Also, cite your claims about "psychology and sociology". Because they are worth going into if they are going to be a part of your argument.
But anyways, this is exactly what politics is about. Politics is about determinating, as a society, what we judge as good and what we judge as bad, and what is the best way to favor the good and combat the evil. You cannot found any public policy without basing yourself on some form of morality.
I base my political policy prescriptions on what is materially best for the society. Also, there is no such thing as good and evil. Life is grey.
I think that cracking down on pimps and brothel owners, to the contrary, has the purpose to emancipate their "employees". As I said, ideally the State should take charge of offering prostitutes opportunities to insert themselves in a normal professional career, but I understand that this is a much broader problem. Nonetheless, while legalizing prostitution might improve the conditions of prostitutes, it does not change the fundamentally degrading nature of prostitution.
The government has been enforcing anti-prostitution laws and has not had your desired effect. As for the bold, if by "might improve the conditions of prostitutes" you mean "it will utterly and massively improve the lot of sex workers" then yes. Every first world nation with legalized prostitution has improved the lot of sex workers dramatically. Legalizing and regulating prostitution has the facts to back it up.
Again your opinion on the matter ultimately boils down to the fact that you just don't like prostitution as a concept personally.