It all stems from what sex is, and what a human is. The Church teaches that the end of sex is procreation. Does this mean that we must try to ensure that a conception arises from every sexual encounter? No.
What must be born in mind here is the distinction between something’s end and its value. What do I mean by this? Allow mw to give an example: the end of a steering wheel is maneuverability. If the wheel does not fulfil this function, then it is worthless and ought to be replaced. In this case, the end and value of the wheel were identical. This is not so for sex.
I have already said that the end of sex is children. The value or meaning of sex, on the other hand, is independant form its end. So things such as the union of the spouses in love, the pleasure, etc. give sex a value that is quite independant from its end. It is because of this that sex between sterile persons, or sex during the woman’s infertile periods is not only permissible, but are an active good.
“So”, one might ask, “since sex has value and meaning independant from its end, then why can’t I use artificial contraception?” If we keep in mind that God invented sex, and that thru sex we become co-creators with God of new life, then to deliberately deprive this sexual encounter of its fertility seems a little sick. (It is “depriving sexual acts of their fertility” that is immoral, not birth control per se. Hence by having sex only during infertile periods one is not not depriving them of fertility, as they havn’t got any fertility of which to deprive them.)
What one is saying to God is “No, I won’t allow you bring anything out of this act.” Just because God doesn’t use every act to spawn new life doesn’t mean that we can pick and choose when we feel like allowing God to work in us. Remember how Jesus told us to be ready in season and out of season? We have to be open to God’s work at all times. Just imagine if the parish priest announced from the pulpit “I’ve decided that for this month we will not let God’s Spirit work in us. If He tries anything, then we’ll stop him.”! This would be nothing short of sacralige.