Objection 1. It would seem there would have been no generation in the state of innocence. For, as stated in Phys. v, 5, "corruption is contrary to generation." But contraries affect the same subject: also there would have been no corruption in the state of innocence. Therefore neither would there have been generation.
Objection 2. Further, the object of generation is the preservation in the species of that which is corruptible in the individual. Wherefore there is no generation in those individual things which last for ever. But in the state of innocence man would have lived for ever. Therefore in the state of innocence there would have been no generation.
Objection 3. Further, by generation man is multiplied. But the multiplication of masters requires the division of property, to avoid confusion of mastership. Therefore, since man was made master of the animals, it would have been necessary to make a division of rights when the human race increased by generation. This is against the natural law, according to which all things are in common, as Isidore says (Etym. v, 4). Therefore there would have been no generation in the state of innocence.
On the contrary, It is written (Genesis 1:28): "Increase and multiply, and fill the earth." But this increase could not come about save by generation, since the original number of mankind was two only. Therefore there would have been generation in the state of innocence.
I answer that, In the state of innocence there would have been generation of offspring for the multiplication of the human race; otherwise man's sin would have been very necessary, for such a great blessing to be its result. We must, therefore, observe that man, by his nature, is established, as it were, midway between corruptible and incorruptible creatures, his soul being naturally incorruptible, while his body is naturally corruptible. We must also observe that nature's purpose appears to be different as regards corruptible and incorruptible things. For that seems to be the direct purpose of nature, which is invariable and perpetual; while what is only for a time is seemingly not the chief purpose of nature, but as it were, subordinate to something else; otherwise, when it ceased to exist, nature's purpose would become void.
Therefore, since in things corruptible none is everlasting and permanent except the species, it follows that the chief purpose of nature is the good of the species; for the preservation of which natural generation is ordained. On the other hand, incorruptible substances survive, not only in the species, but also in the individual; wherefore even the individuals are included in the chief purpose of nature.
Hence it belongs to man to beget offspring, on the part of the naturally corruptible body. But on the part of the soul, which is incorruptible, it is fitting that the multitude of individuals should be the direct purpose of nature, or rather of the Author of nature, Who alone is the Creator of the human soul. Wherefore, to provide for the multiplication of the human race, He established the begetting of offspring even in the state of innocence.
Reply to Objection 1. In the state of innocence the human body was in itself corruptible, but it could be preserved from corruption by the soul. Therefore, since generation belongs to things corruptible, man was not to be deprived thereof.
Reply to Objection 2. Although generation in the state of innocence might not have been required for the preservation of the species, yet it would have been required for the multiplication of the individual.
Reply to Objection 3. In our present state a division of possessions is necessary on account of the multiplicity of masters, inasmuch as community of possession is a source of strife, as the Philosopher says (Politic. ii, 5). In the state of innocence, however, the will of men would have been so ordered that without any danger of strife they would have used in common, according to each one's need, those things of which they were masters--a state of things to be observed even now among many good men.
Source: Summa Theologica (New Advent)