2. Since the intercourse of man and wife carries with it a certain natural shame, those persons should be prevented from such intercourse who owe one another a mutual reverence on account of the tie of blood. And this is the reason touched on in Leviticus xviii.
3. Excessive indulgence in sexual pleasures makes for the corruption of good manners: for such pleasures of all others most absorb the mind and hinder the right exercise of reason. But such excessive indulgence would ensue, if the intercourse of the sexes were allowed among persons who must necessarily dwell under the same roof, where the occasion of such intercourse could not be withdrawn.
5. In human society the widening of friendships is of the first importance. That is done by the marriage tie being formed with strangers.*
It is to be observed that as that inclination is 'natural,' which works upon objects as they usually occur, so law too is framed for what usually happens. Thus it is no derogation from the reasons above alleged, that in some particular case the venture may turn out otherwise: for the good of the individual ought to be overlooked in view of the good of the many, since the good of the multitude is ever more divine than the good of the individual.* Lest however any particular complaint might remain wholly without remedy, there rests with legislators and others on like footing authority to dispense in a general enactment so far as is necessary in a particular case. If the law is human, a dispensation may be given by men possessed of power like to that which made the law. If the law is a divine enactment, a dispensation may be given by divine authority, as in the Old Law a dispensatory indulgence seems to have been granted for plurality of wives, and for concubines, and divorce.*
3.124 : That Marriage ought to be between one Man and one Woman
3.126 : That not all Sexual Intercourse is Sin