Actually the Church says we are not to use artificial birth control methods as each sexual act must be open to conception. That position was explained quite well in Humanae Vitae by Pope Paul VI back in the late sixties. It created quite a firestorm actually, and is actually thought by many to have created or have been instrumental in many of the divisions in the Church today.
I think it all meshes together very well. A single person is not to engage in sex at all and if they do it is a Mortal Sin in almost all cases, A married person is only to have relations with their spouse. If they have sex outside of marriage, mortal sin. True, artificial birth control can stop transmission of some diseases, but you have to engage in sex in order to get them anyway in most circumstances.
The Churchs position has not changed on artificial birth control methods. They are not allowed.