27 There are also they who, in doing open service to evil demons, contain from pleasures of the body, that, through their means, they may satisfy unlawful pleasures, the violence and glow whereof they contain not. Whence also, (to name one case, and pass over the rest in silence by reason of the length of the discourse,) certain come not near even unto their own wives, while, as though clean, they essay through magic arts to gain access unto the wives of others. O marvellous continence, nay rather, singular wickedness and uncleanness!
For, if it were true continence, the lust of the flesh ought rather to contain from adultery, than, in order to commit adultery, from marriage. Forsooth marriage continence is wont to ease this lust of the flesh, and to check its curb but thus far, that neither in marriage itself it run riot by immoderate license, but that a measure be observed, either such as is due to the weakness of the spouse, unto whom the Apostle enjoins not this, as of command, but yields it as of permission; or such as is suited for the begetting of sons, which was formerly the one alone occasion of sexual intercourse to both holy fathers and mothers.
But continence doing this, that is, moderating, and in a certain way limiting in married persons the lust of the flesh, and ordering in a certain way within fixed limits its unquiet and inordinate motion, uses well the evil of man, whom it makes and wills to make perfect good: as God uses even evil men, for their sake whom He perfects in goodness.
Source: On Continence (New Advent)