Paternalism, prostitution and rationality

One argument against prostitution is that it harms prostitutes but the prostitutes are too irrational to realize this so they need the government to protect them by outlawing prostitution. But the government punishes prostitutes by putting them in small metal cages (jail). How is this helping them?

To make the paternalistic argument you have to claim that fear of prison stops some women from becoming prostitutes. But if a woman is rational enough to be deterred by threat of prison from being a prostitute why is she not rational enough to rationally weigh the costs and benefits of becoming a prostitute? The paternalistic argument against prostitution relies on potential prostitutes having not too much, but also not too little rationality.