The reason why the Catholic Church is against contraception is because contraception introduces a willful impediment to the creation of children, which is the very end of marriage. Just as the seed has a purpose to become a tree, a marriage has the purpose of making children, to deny this is to destroy the nature of marriage and to substitute it for mere partnership.
But why doesn't this principle make all non-creative sex evil? I suspect that the answer, as in almost all things Catholic, lies in aristotle especially his On the Generation of Animals. I'll have to read that and continue my analysis in another post.
One should also note that contraception dissolves the meaning and perhaps the existence for the most part, of men and women. This is because if all men and women (as a matter of course) get married, and then use contraception, then since contraception is anti-marriage, by a simple application of modus tollens, it seems that contraception would dissolve the natures of men and women.
But why won't this principle dissolve all males and females, and indeed how can marriage be the end of all males and females, if some monks and nuns exist? Well it seems that both questions share the same answer. It is the case that all human acts are voluntary and what is voluntary happens most of the time but not all the time. So there is some time, it seems, where a human male, does not have to marry and yet is still a male.
And that concludes this post.