The Pope said something to that effect.
I’m not sure he called it a sin though (I read it an hour ago and can’t recall the word “sin”), but I will read more on it.
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“Pope Francis has suggested that couples who prefer pets to children are selfish.
The leader of the world’s 1.3 billion Catholics said substituting pets for children “takes away our humanity”.
During a general audience at the Vatican, he said: “Today… we see a form of selfishness. We see that some people do not want to have a child. Sometimes they have one, and that’s it, but they have dogs and cats that take the place of children. This may make people laugh but it is a reality.”
Pet keeping was “a denial of fatherhood and motherhood and diminishes us, takes away our humanity”, he said. The consequence was that “civilisation grows old without humanity because we lose the richness of fatherhood and motherhood, and it is the country that suffers”.
While saying couples unable to have children for biological reasons could consider adoption, he urged potential parents “not to be afraid” of embarking on parenthood. “Having a child is always a risk, but there is more risk in not having a child,” he said.
Source (The Guardian)”
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Selfishness.
Denial of mother- and fatherhood.
Taking away humanity.
I’m not a fan (at all), but he didn’t (apparently) said it is/was a sin.