Family Planning = Bad, because to the Catholic Church, the main reason for marriage is procreation. Prostitutes = A-OK because they spread disease. Their main purpose isn't to procreate.
The church only permits sex AS procreation- that is why they do not allow birth control. Any sex which CAN NOT result in pregnancy (Masturbation, oral sex, anal sex) is sin.
"There may be justified individual cases, for example when a male prostitute uses a condom, where this can be ... a first bit of responsibility, to re-develop the understanding that not everything is permitted and that one may not do everything one wishes."