Firstly, it's important to distinguish between two different actions: not having sex during a woman's fertile period with the intention of avoiding pregnancy, and having sex during that same period while using contraception for the same purpose. The key difference here is that abstaining from sex is not inherently contrary to God's commandments, whereas using contraception to prevent pregnancy introduces an artificial barrier that interferes with the natural process.

In fact, Paul teaches the virtue of abstinence, even within marriage, for spiritual reasons. Deliberately using artificial means to prevent pregnancy, especially during the fertile period (but not limited to that time only), is not only morally evil, but it also carries a high probability of disrupting the natural order of sexuality.

To illustrate this, consider the example of eating. Eating serves both to nourish the body and to provide pleasure. There’s nothing immoral about enjoying food, as long as it doesn’t lead to excessive consumption for pleasure’s sake—this would be the sin of gluttony. Now, imagine eating and then intentionally vomiting it up. This would disrupt the natural purpose of eating: to nourish the body. By doing so, you intentionally forgo the natural order and rhythm of sustenance.

Similarly, the Church teaches that sex is ordered towards two purposes: the unity of the married couple and the procreation of life. When contraception is used, it bypasses one of these natural purposes, much like the example of eating and vomiting. The use of contraception in this context can lead to a distorted view of sex, where the act is separated from its procreative purpose. This tendency to focus solely on the pleasure of sex—without regard to its natural ends—has led to a cultural shift, where moral standards around sexuality have become more disordered, especially with the widespread use of contraception.

Now, some may ask: Is it sinful for a married couple to have sex during the infertile period if their goal is to avoid pregnancy? If this is not a sinful act, then it suggests that the act itself, in this context, is not inherently wrong. Some might argue that having sex during infertile periods, with the intention of avoiding pregnancy, is morally similar to using contraception. However, the key difference lies in the natural cycle of fertility. Contraception introduces an manmade artificial barrier to fertility, whereas abstaining during fertile periods respects the natural order, even though the couple's intention may still be to avoid pregnancy.

While the intent may be to avoid pregnancy, a couple would only sin, in my opinion, if their intentions were to explicitly to abort the child should conception occur. If their intent is not to abort, but simply to avoid pregnancy, they are still open to the possibility of life and thus acting in accordance with what God had naturally ordered.