23 Lastly, let us hear the Lord Himself delivering most plain judgment on this matter. For, upon His speaking after a divine and fearful manner concerning husband and wife not separating, save on account of fornication, His disciples said to Him, “If the case be such with a wife, it is not good to marry.” To whom He says, “Not all receive this saying. For there are eunuchs who were so born: but there are others who were made by men: and there are eunuchs, who made themselves eunuchs for the sake of the kingdom of heaven: whoso can receive, let him receive.” What could be said more true, what more clear? Christ says, the Truth says, the Power and Wisdom of God says, that they, who of pious purpose have contained from marrying a wife, make themselves eunuchs for the sake of the kingdom of heaven: and against this, human vanity with impious rashness contends, that they, who do so, shun only the present necessity of the troubles of married life, but in the kingdom of heaven have no more than others.
Source: On Holy Virginity (New Advent)