John Chrysostom — as excerpted in the Catena Aurea on Matthew 19:10-12

“When he says, Who have made themselves eunuchs, He does not mean cutting off of members, but a putting away of evil thoughts. For he that cuts off a limb is under a curse, for such an one undertakes the deeds of murderers, and opens a door to Manicheans who depreciate the creature, and cut off the same members as do the Gentiles. For to cut off members is of the temptation of dæmons. But by the means of which we have spoken desire is not diminished but made more urgent; for it has its source elsewhere, and chiefly in a weak purpose and an unguarded heart, For if the heart be well governed, there is no danger from the natural motions; nor does the amputation of a member bring such peacefulness and immunity from temptation as does a bridle upon the thoughts.”