As a Protestant I can assure you that well reasoned arguments (for Protestants it is true that you have to do a little more than cite the catechism) work, regardless if they are by the Catholic Church or not. For example I learned from Catholics that birth control pills potentially can cause abortions and thus when I marry, my wife and I wont use them. However I see nothing wrong with barrier methods and I don’t believe the scriptures teach that such things are wrong. The sin of Onan is the most commonly cited scripture for those who believe all contraception is wrong, but I just don’t see how if you read it in context you can conclude this.
Genesis 38: 6 Judah got a wife for Er, his firstborn, and her name was Tamar. 7 Now Er, Judah’s firstborn, was evil in the Lord’s sight, and the Lord put him to death. 8 Then Judah said to Onan, “Sleep with your brother’s wife. Perform your duty as her brother-in-law and produce offspring for your brother.” 9 But Onan knew that the offspring would not be his; so whenever he slept with his brother’s wife, he released his semen on the ground so that he would not produce offspring for his brother. 10 What he did was evil in the Lord’s sight, so He put him to death also.
In the red portion we see what Onan was required to do and in the bold portion we see why he did not do it, selfishness and pride. Thus it follows logically that the evil referred to is his failure to fulfill his scriptural obligation to his sister in law and brother because of his pride and selfishness. Reading into the passage a general ban on contraception is improper exegesis in my opinion.