Besides, by your own reasoning God can overcome those situations, too. Like the couples who give up trying to conceive, adopt, then shortly thereafter become pregnant. ;)
Pope Pius XI in Casti Connubii wrote:
“Since, therefore, openly departing from the uninterrupted Christian tradition some recently have judged it possible solemnly to declare another doctrine regarding this question, the Catholic Church, ... in order that she may preserve the chastity of the nuptial union from being defiled by this foul stain, ... proclaims anew: any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offense against the law of God and of nature, and those who indulge in such are branded with the guilt of a grave sin. “
Wouldn’t the folk who practice the rhythm method or attempt to determine in any way when the woman would not be fertile be an attempt to have sex without pregnancy as a potential result?
More power, as I say, to the couples who wish to only have intercourse with the view to welcome any resulting pregnancy and abstain otherwise. I find the castigation of responsible family planning to be odd and one result is abortion.
For the good folk practicing the rhythm method, it would surely seem they run afoul of the spirit, if not the letter, of Pius XI’s promulgation above. That’s just an outsider’s view.