Objection 1. It would seem that original sin would be contracted by a person formed miraculously from human flesh. For a gloss on Genesis 4:1 says that "Adam's entire posterity was corrupted in his loins, because they were not severed from him in the place of life, before he sinned, but in the place of exile after he had sinned." But if a man were to be formed in the aforesaid manner, his flesh would be severed in the place of exile. Therefore it would contract original sin.
Objection 2. Further, original sin is caused in us by the soul being infected through the flesh. But man's flesh is entirely corrupted. Therefore a man's soul would contract the infection of original sin, from whatever part of the flesh it was formed.
Objection 3. Further, original sin comes upon all from our first parent, in so far as we were all in him when he sinned. But those who might be formed out of human flesh, would have been in Adam. Therefore they would contract original sin.
On the contrary, They would not have been in Adam "according to seminal virtue," which alone is the cause of the transmission of original sin, as Augustine states (Gen. ad lit. x, 18, seqq.).
I answer that, As stated above (1,3), original sin is transmitted from the first parent to his posterity, inasmuch as they are moved by him through generation, even as the members are moved by the soul to actual sin. Now there is no movement to generation except by the active power of generation: so that those alone contract original sin, who are descended from Adam through the active power of generation originally derived from Adam, i.e. who are descended from him through seminal power; for the seminal power is nothing else than the active power of generation. But if anyone were to be formed by God out of human flesh, it is evident that the active power would not be derived from Adam. Consequently he would not contract original sin: even as a hand would have no part in a human sin, if it were moved, not by the man's will, but by some external power.
Reply to Objection 1. Adam was not in the place of exile until after his sin. Consequently it is not on account of the place of exile, but on account of the sin, that original sin is transmitted to those to whom his active generation extends.
Reply to Objection 2. The flesh does not corrupt the soul, except in so far as it is the active principle in generation, as we have stated.
Reply to Objection 3. If a man were to be formed from human flesh, he would have been in Adam, "by way of bodily substance" [The expression is St. Augustine's (Gen. ad lit. x). Cf. Summa Theologica TP, 31, 6, Reply to Objection 1, but not according to seminal virtue, as stated above. Therefore he would not contract original sin.
Source: Summa Theologica (New Advent)