It may be accurate, though picayune, to insist that Pope Francis did not say, *breed* like rabbits, as the phrase typically is attested. Be...breed...whatever.
This post of Dr. Gregory Popčak is informative, especially his reference to paragraph 50 of the Vatican II document Gaudium et Spes.
One segment of the citation clears up this breeding problem (emphases mine):
...in their manner of acting, spouses should be aware that they cannot proceed arbitrarily, but must always be governed according to a conscience dutifully conformed to the divine law itself, and should be submissive toward the Church’s teaching office, which authentically interprets that law in the light of the Gospel.Before reading the Popčak piece, I recalled a phrase that I picked up at some point in the seminary: humano modo. Context? Glad you asked:
Can. 1061 §1. A valid marriage between the baptized is called ratum tantum if it has not been consummated; it is called ratum et consummatum if the spouses have performed between themselves in a human fashion [humano modo] a conjugal act which is suitable in itself for the procreation of offspring, to which marriage is ordered by its nature and by which the spouses become one flesh. {source}A "human manner"--a manner suitable to free and rational creatures who are "now called children of God, for that is what we are" (1 Jn 3:1). A human manner presumes a total investment of self that is permanent, exclusive, and open to new life. A human manner is not "arbitrary" and casual, fit for public display and risible observation.
Can rabbits marry each other--or dogs, cats, gerbils, or even the most intelligent orangutans? Does a total investment of self that is permanent, exclusive, and open to new life ("openness" being a uniquely human possibility) even occur to rabbits, or any of the other animals? The sexual expression of rabbits and other animals is instinctual, not free and rational.
Now maybe scientists and others have observed in animals some approximation to human love. Every concern for the other as other certainly participates in, derives from, divine love--and cannot otherwise exist. But we are not animals; and Pope Francis is reminding us that the Catholic Church wants responsible parents who must decide wisely and generously how they will cooperate with God's gift of generation. Indiscriminate copulation, devoid of devotion, will not suffice.
Maybe I'm a "speciesist" by insisting that, however tender we may deem it to be, the procreation and rearing of animals is different from human love both in degree and in kind. Maybe my take doesn't catch the spirit of the Holy Father's words any more than the rereading by any author in the mainstream media (or even this article), but I offer it nonetheless.