Why does Mami say "Good bulls sire cows"?

Although Papi seems to regret having no sons, he likes to say, "Good bulls sire cows," and he is a devoted father to his daughters. He seems to be referring to the cultural bias of having sons being more special than having girls. However, his social conservatism and patriarchal convictions contribute to considerable strife between himself and his Americanized, feminist daughters.