12 Let marriages possess their own good, not that they beget sons, but that honestly, that lawfully, that modestly, that in a spirit of fellowship they beget them, and educate them, after they have been begotten, with cooperation, with wholesome teaching, and earnest purpose: in that they keep the faith of the couch one with another; in that they violate not the sacrament of wedlock. All these, however, are offices of human duty: but virginal chastity and freedom through pious continence from all sexual intercourse is the portion of Angels, and a practice, in corruptible flesh, of perpetual incorruption. To this let all fruitfulness of the flesh yield, all chastity of married life; the one is not in (man's) power, the other is not in eternity; free choice has not fruitfulness of the flesh, heaven has not chastity of married life. Assuredly they will have something great beyond others in that common immortality, who have something already not of the flesh in the flesh.
Source: On Holy Virginity (New Advent)