Thomas Aquinas

“ Therefore matrimony does not come under a precept.
I answer that, Nature inclines to a thing in two ways. In one way as to that which is necessary for the perfection of the individual, and such an obligation is binding on each one, since natural perfections are common to all. In another way it inclines to that which is necessary for the perfection of the community; and since there are many things of this kind, one of which hinders another, such an inclination does not bind each man by way of precept ”

“ For if the motive for the marriage act be a virtue, whether of justice that they may render the debt, or of religion, that they may beget children for the worship of God, it is meritorious. But if the motive be lust, yet not excluding the marriage blessings, namely that he would by no means be willing to go to another woman, it is a venial sin; while if he exclude the marriage blessings, so as to be disposed to act in like manner with any woman, it is a mortal sin. ”

“ Properly speaking, a thing is said to be excused when it has some appearance of evil, and yet is not evil, or not as evil as it seems, because some things excuse wholly, others in part. And since the marriage act, by reason of the corruption of concupiscence, has the appearance of an inordinate act, it is wholly excused by the marriage blessing, so as not to be a sin. ”

“ First, in relation to the principal end of matrimony, namely the good of the offspring. For nature intends not only the begetting of offspring, but also its education and development until it reach the perfect state of man as man, and that is the state of virtue. ”

“ The difficulty required for merit of the accidental reward is a difficulty of labor, but the difficulty required for the essential reward is the difficulty of observing the mean, and this is the difficulty in the marriage act. ”

“ Further, those things without which the intention of nature can be maintained would seem not to be natural. But nature intends the preservation of the species by generation which is possible without matrimony, as in the case of fornicators. ”

“ We are united to God by the habit of grace and by the act of contemplation and love. Therefore whatever severs the former of these unions is always a sin, but not always that which severs the latter, since a lawful occupation about lower things distracts the mind so that it is not fit for actual union with God ”

“ Secondly, in relation to the secondary end of matrimony, which is the mutual services which married persons render one another in household matters. For just as natural reason dictates that men should live together, since one is not self-sufficient in all things concerning life, for which reason man is described as being naturally inclined to political society, so too among those works that are necessary for human life some are becoming to men, others to women. ”

“ And it is owing to this difference, as well as to Divine providence which governs all, that one person chooses one office such as husbandry, and another person another. And so it is too that some choose the married life and some the contemplative. ”

“ Accordingly man's nature inclines to matrimony on the part of the difference, as regards the second reason given above; wherefore the Philosopher (Ethic. viii, 11,12; Polit. i) gives this reason in men over other animals; but as regards the first reason it inclines on the part of the genus; wherefore he says that the begetting of offspring is common to all animals. Yet nature does not incline thereto in the same way in all animals; since there are animals whose offspring are able to seek food immediately after birth, or are sufficiently fed by their mother ”

“ Although they are the same as to their natural species, they differ as to their moral species, which differs in respect of one circumstance, namely intercourse with one's wife and with another than one's wife; just as to kill a man by assault or by justice differentiates the moral species, although the natural species is the same ”

“ Now a child cannot be brought up and instructed unless it have certain and definite parents, and this would not be the case unless there were a tie between the man and a definite woman and it is in this that matrimony consists. ”