Damnit, I love the RCC. That's the overt reasoning; maybe they really believe it. Sociochemically speaking, however, this is how I see it. (Is it bullshit? You decide!)

The 'anti-contraception' meme is a part of the 'Catholic Dogma' meme complex. This part contributes to the propagation of the complex over time, thus reinforcing itself as an idea and the church as a whole. Simply stated, the idea spreads itself, causing it (at least if considered in isolation), to become more and more prevalent over time. It does so by the following mechanism:

1. Decrease in contraceptive use --> Increase in Catholic children. (Biological Step)
2. Meme is transferred along the extremely effective (Parent--> Child) indoctrination route. (Memetic Step)

Seeing as the Parent to Child transfer has an extremely high percentage yield, the rate of reproduction is the limiting step in this reaction.

This meme has the opportunity to be at least doubly successful, seeing as it is positively reinforced by other propagation mechanisms bound into the 'Catholic Dogma' complex, such as the Sex Guilt Mechanism (Which I'll node later, unless you want to). Provided two propagation memes are logically consistent, the stability of the thought structure will not diminish over time.