That's the thing, it's ok if you do.

Maybe sometimes you need to, even if you are in a good relationship. And the prostitute wants to make money.

But privacy is paramount for this to work.

No different than lets say hair dressing, really. A marriage will not work if one is poor eating water and bread and the other one is rich and can indulge oneself. At the end of the day, money is for mutual benefit.