14 Wherefore I cannot indeed say, of females who have fallen away from a better purpose, in case they shall have married, that they are adulteries, not marriages; but I plainly would not hesitate to say, that departures and fallings away from a holier chastity, which is vowed unto the Lord, are worse than adulteries. For if, what may no way be doubted, it pertains unto an offense against Christ, when a member of Him keeps not faith to her husband; how much graver offense is it against Him, when unto Himself faith is not kept, in a matter which He requires when offered, Who had not required that it should be offered.
For when each fails to render that which, not by force of command, but by advice of counsel, he vowed, by so much the more does he increase the unrighteousness of the wrong done to his vow, by how much the less necessity he had to vow. These matters I for this reason treat of, that you may not think either that second marriages are criminal, or that any marriages whatsoever, being marriages, are an evil. Therefore let this be your mind, not that you condemn them, but that you despise them.
Therefore the good of widowed chastity is becoming after a brighter fashion, in that in order to make vow and profession of it, females may despise what is both pleasing and lawful. But after profession of vow made they must continue to rein in, and overcome, what is pleasing, because it is no longer lawful.
Source: On the Good of Widowhood (New Advent)