Because you are subverting gods will? Seems like if god wanted you to have a baby bad enough he could simply cause the BC to fail right? Fortunately today most Catholics seem to realize that they don't want or need 17 children.
"In A.D. 195, Clement of Alexandria wrote, "Because of its divine institution for the propagation of man, the seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted" (The Instructor of Children 2:10:91:2). " http://www.catholic.com/library/Birth_Control.asp
But wasting it (i.e. not making little Catholics with every single sperm) is a biological certainty whether you have sex and conceive or remain celibate.
And if it's Gods will which sperm/egg gets lucky what did he make all the others for?
The immaculate conception demonstrates that God wasn't bound by the apparent laws of genetics and sexual reproduction anyway so why aren't we all smooth like Ken and Barbie? It's not like he needed to give us all that dangly stuff.
Conception could be caused spontaneously by cutting the wedding cake - so much less "wasteful".
Along with what others have said I want to add that the more people that are born into a particular religious sect means more soldiers for the army of god. It is a lot easier to convince someone to fly a plane into a building if they have been brainwashed from birth.
I couldn't tell why really. For which I can tell, the biggest reason is because sex should be only for making babies (of course in the context of sacred marriage); then, in their freaked out minds, using a condom or whatever is going against that concept...maybe they think love=having a dozen kids....is so obvious that those concepts were "right" 2000 thousand years ago, but not now!
About the holyness, they believe the souls is "Infused"; God puts it in the zygote once the sperm and the ovule are united. I don't think sperm is sacred for them, but the zygote is. Abortion, then, is killing something god made sacred. yuuuk
It's my understanding that before microscopes it was thought that there were little people in male ejaculate and that the female was simply the repository garden in which to grow it. This also formed the basis of rules against male masturbation. They thought males were ejaculating live humans.
Unborn children, er, babies, oops foetuses, so hard to get it right these days...on a bare emotional level I can understand the call to defend. I have my own problems with abortion myself. Throw in a load of scripture related to the issue of abortion by virtue of the fact they seem to mention pregnancy and nothing else - presto you have the conventional Christian anti-abortion stance.
I don't really agree that the sole purpose is to create more 'Catholics', maybe if the family was Catholic.