We must seek a solution from what has been said before: for it has been said (Chapp. XVI, LXIV) that God has care of everything according to that which is good for it. Now it is good for everything to gain its end, and evil for it to be diverted from its due end. But as in the whole so also in the parts, our study should be that every part of man and every act of his may attain its due end. Now though the semen is superfluous for the preservation of the individual, yet it is necessary to him for the propagation of the species: while other excretions, such as excrement, urine, sweat, and the like, are needful for no further purpose: hence the only good that comes to man of them is by their removal from the body. But that is not the object in the emission of the semen, but rather the profit of generation, to which the union of the sexes is directed. But in vain would be the generation of man unless due nurture followed, without which the offspring generated could not endure. The emission of the semen then ought to be so directed as that both the proper generation may ensue and the education of the offspring be secured.
Hence it is clear that every emission of the semen is contrary to the good of man, which takes place in a way whereby generation is impossible; and if this is done on purpose, it must be a sin. I mean a way in which generation is impossible in itself as is the case in every emission of the semen without the natural union of male and female: wherefore such sins are called 'sins against nature.' But if it is by accident that generation cannot follow from the emission of the semen, the act is not against nature on that account, nor is it sinful; the case of the woman being barren would be a case in point.
Likewise it must be against the good of man for the semen to be emitted under conditions which, allowing generation to ensue, nevertheless bar the due education of the offspring. We observe that in those animals, dogs for instance, in which the female by herself suffices for the rearing of the offspring, the male and female stay no time together after the performance of the sexual act. But with all animals in which the female by herself does not suffice for the rearing of the offspring, male and female dwell together after the sexual act so long as is necessary for the rearing and training of the offspring. This appears in birds, whose young are incapable of finding their own food immediately they are hatched: for since the bird does not suckle her young with milk, according to the provision made by nature in quadrupeds, but has to seek food abroad for her young, and therefore keep them warm in the period of feeding, the female could not do this duty all alone by herself: hence divine providence has put in the male a natural instinct or standing by the female for the rearing of the brood. Now in the human species the female is clearly insufficient of herself for the rearing of the offspring, since the need of human life makes many demands, which cannot be met by one parent alone. Hence the fitness of human life requires man to stand by woman after the sexual act is done, and not to go off at once and form connexions with any one he meets, as is the way with fornicators. Nor is this reasoning traversed by the fact of some particular woman having wealth and power enough to nourish her offspring all by herself: for in human acts the line of natural rectitude is not drawn to suit the accidental variety of the individual, but the properties common to the whole species.*
A further consideration is, that in the human species the young need not only bodily nutrition, as animals do, but also the training of the soul. Other animals have their natural instincts (suas prudentias) to provide for themselves: but man lives by reason, which [read quam] takes the experience of a long time to arrive at discretion. Hence children need instruction by the confirmed experience of their parents: nor are they capable of such instruction as soon as they are born, but after a long time, the time in fact taken to arrive at the years of discretion. For this instruction again a long time is needed; and then moreover, because of the assaults of passion, whereby the judgement of prudence is thwarted, there is need not of instruction only, but also of repression. For this purpose the woman by herself is not competent, but at this point especially there is requisite the concurrence of the man, in whom there is at once reason more perfect to instruct, and force more potent to chastise. Therefore in the human race the advancement of the young in good must last, not for a short time, as in birds, but for a long period of life. Hence, whereas it is necessary in all animals for the male to stand by the female for such time as the father's concurrence is requisite for bringing up of the progeny, it is natural for man to be tied to the society of one fixed woman for a long period, not a short one. This social tie we call marriage. Marriage then is natural to man, and an irregular connexion outside of marriage is contrary to the good of man; and therefore fornication must be sinful.
Nor yet should it be counted a slight sin for one to procure the emission of the semen irrespective of the due purpose of generation and rearing of issue, on the pretence that it is a slight sin, or no sin at all, to apply any part of one's body to another use than that to which it is naturally ordained, as if, for example, one were to walk on his hands, or do with his feet something that ought to be done with his hands. The answer is that by such inordinate applications as those mentioned the good of man is not greatly injured: but the inordinate emission of the semen is repugnant to the good of nature, which is the conservation of the species.* Hence, after the sin of murder, whereby a human nature already in actual existence is destroyed, this sort of sin seem to hold the second place, whereby the generation of human nature is precluded.
The above assertions are confirmed by divine authority. The unlawfulness of any emission of semen, upon which offspring cannot be consequent, is evident from such texts as these: Thou shalt not lie with mankind as with womankind: Thou shalt not lie with any beast (Levit. xviii, 22, 23): Nor the effeminate, nor sodomites, shall possess the kingdom of God (1 Cor. vi, 10). The unlawfulness of fornication and of all connexion with any other woman than one's own wife is clear from Deut. xxiii, 17: There shall be no whore among the daughters of Israel, nor whoremonger among the sons of Israel: Keep thyself from all fornication, and beyond thine own wife suffer not the charge of knowing another (Job. iv, 13): Fly fornication (1 Cor. vi, 18).
Hereby is refuted the error of those who say that there is no more sin in the emission of the semen than in the ejection of other superfluous products from the body.
3.121 : That the Divine Law directs man to a Rational Use of Corporeal and Sensible Things
3.123 : That Marriage ought to be Indissoluble