I'm completely sympathetic to that view, but if the person has the choice of prostituting her/himself or starve, and you ban the first option, what so you think will happen?

If you want to prevent these situation you're describing, you need to give people a third option, that enables them to avoid the first two. Criminalizing prostitution will just harm those you want to help.

EDIT: from your other post, it seems we're in agreement.