In many countries prostitution is as legal as software development. I think we should refrain from these jokes, especially since most do it out of a lack of alternative careers and to feed families.

I read it completely the opposite. The son looks down at the prostitute, but the father sees humanity in her and does not judge and treats her as a fellow person with dignity.

But the father says she is no lady

The father says that her status does not determine his behavior as a gentleman.

To belabor the point, being an ethical person is not contingent on ethical behavior by the other party.

As much as I agree with the point you're making, there's nothing wrong with being a prostitute.