The problem is that we live in a highly materialistic and consumerist society. Indeed, the paradox where poor families have more children than those with means like middle and upper middle class families is unique to consumerist societies. The reason should be clear. Having the extra income relative to the poor, many prioritize spending what they've made on goods and services that contribute to their comfort, pleasure, and "status" (keeping up with the Jones') over children who often require sacrificing comfort and pleasure. Having children is no longer seen as a demanding privilege, but a pointless burden.
Now, nothing is wrong with material goods as such. Nothing is wrong with sacrificing having children for the sake of a higher goal (as a Catholic, you already understand that to be something people can do, e.g., the priesthood or the religious life). But a society that has traded the privilege of raising a new generation in not for superior goods but for inferior goods, for the sake of empty self-indulgence, surely smacks of malaise and decadence. Is it any wonder why so many of us are depressed when we celebrate a perverted form of individualism that is ultimately self-destructive? We have a nature and this recent fad that denies human nature or perverts it will not produce happiness.