Why can't Catholics use birth control?

The Whole Church teaches that Birth Control is wrong altogether. It contradicts (Notice contra- ceptives, contradicting with conception) with the natural cycle and does not respect the gift of sex. Even condoms, even if they are used to prevent HIV are not approved by the Church (Although it is not a sin to use it in moral sex), if they can't even effectively stop sperm, how can it stop HIV which is a virus smaller than sperm. Pills also harm the body and thus is a sin also against health.

It also reduces ones virtues (Getting sex whenever you want it is not a virtue).

A better alternative (Which is approved by the Catholic and is morally sound if used properly) is NFP (Natural Family Planning). It is very effective and it is a moral form of birth control. It respects the natural cycles that God made our bodies with. By collecting information about the body, one can know when the woman is fertile of infertile (Only in certain periods a month can a woman get a child through sex). It helps build virtues such as patience.

There is a big difference between "I want sex right now" (Contraceptives), and "It would not be responsible it do it right now since we don't have enough money to support another child, let us wait till Tuesday" (NFP)

Yes, the entire Church opposes birth control (Except for NFP), but there are some Catholics that reject the teachings of the Church.