Oh boy, what have I stumbled upon... The purpose of sex is found in marriage only. If you're looking outside of marriage, you would be confused by the Catholic understanding of it. It's for marrieds only. It is for procreation and unity of the married couple, not just procreation, not just unity but both. And it's not "anything goes" as long as it's consensual either. A lot of crazy logic and confused morality has been thrown around here.
The Catholic POV is knowing what marriage and sex is for. Catholic theology so, so lofty and deep no one could possible explain this all in a debate forum setting such as this. It takes studying it, certainly not debating it, but here it is in a nut shell. It's all about family. God Himself is not a loner up there in heaven but a He is a family. Family is the beginning and end of it all. It is the foundation and the core, root of life itself and our existence. Family is the human race so that is why God made for man a woman and they are to multiply. Sex and marriage is about self giving, giving all of ourselves holding nothing back. The self giving love of man and woman and the unity that creates is expressed in sex where man and woman become one by giving all of themselves (even fertility) to the other. They become one and just as God is one God but a family (Father, Son, Spirit), so is the couple and that bond of married love results in a child. That is the image of God (who is the Holy Trinity) we are created in. And just as God gave all of Himself holding nothing back to us, we are to give to one another the same total self giving, holding nothing back. So that is why contraception is wrong.
The arguments against this put fourth here is what happens when you take sex outside of marriage. We are talking about sex in marriage. Anything other than that is sin. It's like a fire inside your fireplace. Its warm, gives light, heats your home but take that out of it's fireplace and put it where it does not belong (middle of your living room) and it burns your house down. The sex that happens outside marriage is the cause of problems and abuse. The sin is lust and that is the cause of abortion and abuse, people just not respecting sex being for marriage only between man and woman. Contraception is to take away the God designed, natural result of sex which would be a child. Sex outside of marriage is just reckless and dangerous, like a fire that burns outside of your fireplace.
No one here should deny that God designed sex to result in a child because it does. Contraception and abortion is to erase that result. Period. There is nothing noble behind that. It is what it is. To rid one self of the natural results so to rid one self of any consequence of doing what you are not supposed to be doing if you are engaging in sex outside of a loving man/woman marriage.
To alter God's design is why it is sinful. God design men to be fertile 100% of the time but not women. So sex was made for procreation but it is not only for procreation because women are not always fertile and will not always be through out her life like men will. Sex is also to be in union, to bond, to be one with each other. Women are not always going to be able to conceive so if there are grave reasons a couple should not have a child they certainly can be celibate but if the marriage can not withstand that then they can work with nature (God) to not conceive. Not thwart Him with contraception. And God can not be stopped no, He can't but God is not going to interfere with our free willed human choices just as He did not interfere with Adam and Eve's choice to disobey Him. If man being so smart with the intelligence God gave him wants to be a god and create for himself contraception, good for him, God isn't going to stop him, he can sin all he wants, it's called free will. God is not going to impose His will on us in that type of context. He allows us to choose for ourselves. But God is there with mercy and forgiveness when we choose not His will but our own. It's just a cop out to say, If God wants a baby, latex won't stop Him and Married couple can do what they want in their bedrooms." He design the whole thing the way He did and that is His will, that we follow it His way. If you don't, you don't. God does allow man to go his own way even to the point he changes God's creation. One of these days He'll have enough and come again.
So, it's all about the means we use and the intentions behind it. It's not always sinful to prevent a pregnancy for grave reasons like heath issues or poverty but the means we use must be according to God's creative design. Contraception and abortion is certainly not it.