From this link, I've found this
all these women shall be wives in common to all the men, and not one of them shall live privately with any man; the children too should be held in common so that no parent shall know which is his own offspring, and no child shall know his parent.” This belief is associated with a need for eugenics, as “the best men must cohabit with the best women in as many cases as possible and the worst with the worst in the fewest, and that the offspring of the one must be reared and that of the other not, if the flock is to be as perfect as possible.” More pernicious still is his prescription for infanticide: “The offspring of the inferior, and any of those of the other sort who are born defective, they will properly dispose of in secret, so that no one will know what has become of them. That is the condition of preserving the purity of the guardians’ breed.”
Does this block appear in The Republic? Did Plato criticize or reject institution of marriage?