I seem to be posting this response twice a day. Your interlocutors believe in God. Are they Christians? Because if they are Christians (especially if they are “Bible Christians”), they might respond to a scriptural presentation on natural law. I paste this in my response on another thread:
The Catholic position on contraception (as opposed to abstinence) derives from the fundamental understanding of the dignity of the human person as being greater than his animal nature, animated by a spark of the divine. The Church has always taught that the marital embrace is a holy act; it is where a man and a woman cooperate with God in creation. We are pro-creators.
Non-Catholics often “require” a scriptural argument or may feel that something is not specifically laid out in Scripture, it is up for grabs. But the case against contraception *does *have strong scriptural support.
In the first chapter of the Epistle to the Romans, Paul mounts the “natural law” argument. Even the barbarians, he says, have no excuse for failing to heed the law written on the human heart – the law which does not require special divine revelation. Thus, Paul equates unnatural sexual relations with idolatry. Although he specifies homosexual acts here, he uses the term “unnatural relations.” Contraception is, inarguably, unnatural. (Duh!)
Romans, Chapter I (RSV – Protestant edition – emphasis added):
14] I am under obligation both to Greeks and to barbarians, both to the wise and to the foolish:
15] so I am eager to preach the gospel to you also who are in Rome.
. . .
17] For in it the righteousness of God is revealed through faith for faith; as it is written, “He who through faith is righteous shall live.”
18] For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth.
19] For what can be known about God is plain to them i.e., the Greeks and Barbarians], because God has shown it to them.
20] Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse;
21] for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened.
22] Claiming to be wise, they became fools,
23] and exchanged the glory of the immortal God for images resembling mortal man or birds or animals or reptiles.
24] Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves,
25] because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed for ever! Amen.
26] For this reason God gave them up to dishonorable passions. Their women exchanged natural relations for unnatural,
27] and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in their own persons the due penalty for their error.
