There is no chapter and verse to back it up. It's just more of the same archaic killjoy "if I don't get any, then no one gets any" nonsense spewed forth from underneath the papal baldachin.
Wake up Ratzinger! The marriage bed is undefiled! Hebrews 13:4

Most arguments against birth control are based on the story of Onan and his brothers.

Yep, it's ridiculous. Gotta love the religious movement that prefers to allow people to die so they can pretend sex doesn't exist outside Christian marriages.

Faiths evolve and separate into different branches.
Some branches flourish.
But this branch could be a cul de sac.

I'm not Catholic, so if I get this wrong, I apologize.

But as I understand it the idea is that sexual activity has one purpose, and one purpose only -- to create children. So having sex while employing birth control is seen as frustrating the basic purpose of the act.

I don't agree with this, by the way, but I like to understand the arguments of people who disagree with me.

The Roman Catholic Church has always sustained that sexual relations should only be for reproductive purposes, never the less they accept the rhythm method which is practiced by 96% of married Roman Catholics. Everything in life has an exception even by the strict Roman Catholic Church.

If a woman ovulates twice a month she can not follow the rhythm method or she will become pregnant no matter what. Lets say another couple can not have more children because the wifes health is very poor or the husband has some illness and his life expectancy is short, or any of the two have some hereditary trait that can be passed on to the children. In these cases one can and should obtain a dispense from the church and be allowed the use of contraceptives, this is done in absolute silence and great secrecy. I know of several cases as the ones described and they got their dispense.

Of course the idea is not to publish it or send a video to CNN, it is about doing what we are told but in absolute secrecy. The same goes for abortion, the church when addressing the masses will always say no, but there are great exceptions dealt with on a one by one case basis. The church will always help us if we do the right things at the right time and for the right reasons.

Not all teachings are based on scripture. There are 2 sources of truth, revelation (scripture) and reason. The Church teachings on human sexuality are based upon rational reflection, the love of the human person and the appropriate expression of human sexuality that brings about happiness for that person.