Objection 1. It would seem that generation by coition would not have existed in the state of innocence. For, as Damascene says (De Fide Orth. ii, 11; iv, 25), the first man in the terrestrial Paradise was "like an angel." But in the future state of the resurrection, when men will be like the angels, "they shall neither marry nor be married," as is written Matthew 22:30. Therefore neither in paradise would there have been generation by coition.
Objection 2. Further, our first parents were created at the age of perfect development. Therefore, if generation by coition had existed before sin, they would have had intercourse while still in paradise: which was not the case according to Scripture (Genesis 4:1).
Objection 3. Further, in carnal intercourse, more than at any other time, man becomes like the beasts, on account of the vehement delight which he takes therein; whence contingency is praiseworthy, whereby man refrains from such pleasures. But man is compared to beasts by reason of sin, according to Psalm 48:13: "Man, when he was in honor, did not understand; he is compared to senseless beasts, and is become like to them." Therefore, before sin, there would have been no such intercourse of man and woman.
Objection 4. Further, in the state of innocence there would have been no corruption. But virginal integrity is corrupted by intercourse. Therefore there would have been no such thing in the state of innocence.
On the contrary, God made man and woman before sin (Genesis 1-2). But nothing is void in God's works. Therefore, even if man had not sinned, there would have been such intercourse, to which the distinction of sex is ordained. Moreover, we are told that woman was made to be a help to man (Genesis 2:18-20). But she is not fitted to help man except in generation, because another man would have proved a more effective help in anything else. Therefore there would have been such generation also in the state of innocence.
I answer that, Some of the earlier doctors, considering the nature of concupiscence as regards generation in our present state, concluded that in the state of innocence generation would not have been effected in the same way. Thus Gregory of Nyssa says (De Hom. Opif. xvii) that in paradise the human race would have been multiplied by some other means, as the angels were multiplied without coition by the operation of the Divine Power. He adds that God made man male and female before sin, because He foreknew the mode of generation which would take place after sin, which He foresaw. But this is unreasonable. For what is natural to man was neither acquired nor forfeited by sin. Now it is clear that generation by coition is natural to man by reason of his animal life, which he possessed even before sin, as above explained (97, 3), just as it is natural to other perfect animals, as the corporeal members make it clear. So we cannot allow that these members would not have had a natural use, as other members had, before sin.
Thus, as regards generation by coition, there are, in the present state of life, two things to be considered. One, which comes from nature, is the union of man and woman; for in every act of generation there is an active and a passive principle. Wherefore, since wherever there is distinction of sex, the active principle is male and the passive is female; the order of nature demands that for the purpose of generation there should be concurrence of male and female. The second thing to be observed is a certain deformity of excessive concupiscence, which in the state of innocence would not have existed, when the lower powers were entirely subject to reason. Wherefore Augustine says (De Civ. Dei xiv, 26): "We must be far from supposing that offspring could not be begotten without concupiscence. All the bodily members would have been equally moved by the will, without ardent or wanton incentive, with calmness of soul and body."
Reply to Objection 1. In paradise man would have been like an angel in his spirituality of mind, yet with an animal life in his body. After the resurrection man will be like an angel, spiritualized in soul and body. Wherefore there is no parallel.
Reply to Objection 2. As Augustine says (Gen. ad lit. ix, 4), our first parents did not come together in paradise, because on account of sin they were ejected from paradise shortly after the creation of the woman; or because, having received the general Divine command relative to generation, they awaited the special command relative to time.
Reply to Objection 3. Beasts are without reason. In this way man becomes, as it were, like them in coition, because he cannot moderate concupiscence. In the state of innocence nothing of this kind would have happened that was not regulated by reason, not because delight of sense was less, as some say (rather indeed would sensible delight have been the greater in proportion to the greater purity of nature and the greater sensibility of the body), but because the force of concupiscence would not have so inordinately thrown itself into such pleasure, being curbed by reason, whose place it is not to lessen sensual pleasure, but to prevent the force of concupiscence from cleaving to it immoderately. By "immoderately" I mean going beyond the bounds of reason, as a sober person does not take less pleasure in food taken in moderation than the glutton, but his concupiscence lingers less in such pleasures. This is what Augustine means by the words quoted, which do not exclude intensity of pleasure from the state of innocence, but ardor of desire and restlessness of the mind. Therefore continence would not have been praiseworthy in the state of innocence, whereas it is praiseworthy in our present state, not because it removes fecundity, but because it excludes inordinate desire. In that state fecundity would have been without lust.
Reply to Objection 4. As Augustine says (De Civ. Dei xiv, 26): In that state "intercourse would have been without prejudice to virginal integrity; this would have remained intact, as it does in the menses. And just as in giving birth the mother was then relieved, not by groans of pain, but by the instigations of maturity; so in conceiving, the union was one, not of lustful desire, but of deliberate action."
Source: Summa Theologica (New Advent)