There are a lot of myths and misconceptions about what the Church teaches about sex. Lets clear those out of the way before we can discuss more of the specifics.
Sex is good. Not many people think the Church teaches that, but in fact, She does. She does so emphatically! The Church teaches that the act of intercourse actually is a foretaste of Heaven - that is, sex feels so wonderful that it should make us think "how awesome is Heaven going to be if Earth is this good??" Even more, the Church teaches that sex parallels the reception of communion (the Holy Eucharist, who is Jesus Christ himself). Why? Well, because as we learn in St. Paul's letter to the Ephesians that marriage is a symbol of the love between "Christ and his Church." The consummation of the love between Christ and his Bride, the Church, is the reception of communion - that is where two become one. And in marriage, the act where two become one, is sex. THAT is how good we teach sex is!
Sex is precious. Because sex is such a great gift, and is so powerful as to completely unite two people, the Church teaches sex is precious. Because it's precious, it is something to be protected and to protect people from. I know that sounds odd, but to use a popular example - fire is a great gift! But in the wrong circumstances, it could hurt people. We teach the same about sex and everyone knows this to be empirically true - in the wrong context, used by the wrong people, it can be very hurtful. Because of this, the Church teaches that -
Sex is only appropriate in ONE context - between married persons who are open to the possibility of having a child. If this doesn't describe you, then the Church teaches you shouldn't have sex - not because she's a big meanie, but because it isn't good for you.
But even this 'simple' concept is so radical for our times that it needs some unpacking. So lets set to it.
First off, you have to be married. Sex belongs in a permanent union because it is meant to bond people together permanently. Remember, the Church also teaches that divorce is wrong and that marriage forever - the reality that we face today of no-fault divorce is not the vision of the Church. She intends for marriage to be forever. Sex, as a foretaste of Heaven (an eternal - forever - reality), belongs within that permanent bond.
Secondly, you have to be open to having a child. This is NOT the same thing as wanting to get pregnant, or only having sex to get pregnant. It's saying that you understand that a baby could result from sex and that is a possibility you're willing to accept. Some people think this means that Catholics don't 'believe' in birth control; that is a false statement for three reasons: 1) we believe in birth control - as in, we know it exists, we don't think it's a unicorn; 2) we actually do think using birth control is licit; 3) contraception is what we know to be immoral.
Contraception - to be contra (against) conception is what we disagree with. We understand that married couples may need to abstain from having children, either permanently or for a time. These decisions are discerned by married couples with the advice of a holy priest. For Catholics who discern a need to abstain, they are free to, using Natural Family Planning (NFP). This term is used to describe broadly several methods, all of which do the same thing: track the woman's cycle, determining fertile periods, and abstaining from sex for those times.
Next time, we'll talk more about NFP and address some specific questions about that. Hang on - almost done with this groundwork stuff!
For Part I, see here.