Thomas Aquinas On Original Sin, Part 4.

From Summa Theologica:

Treatise on Habits in Particular, Question 81, Article 4: Whether original sin would be contracted by a person formed miraculously from human flesh?

Obviously, the above question is raised due to questions over the sinlessness of Christ. Apparently, there were some thinkers in Aquinas' day that would have answered "yes" to the above question due to a gloss on Genesis 4:1 (that is, due to a church tradition) or due to the idea that the soul has become infected by the flesh. Aquinas answers both of these objections to Christ's sinfulness. He also answers a third objection, which objection seems to find more basis in the biblical account, namely:

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.

Aquinas gives a philosophical answer in which he connects original sin with a willing intention of human individuals- apparently if there was no intention to procreate (or, one may assume, to engage in any action that may lead to procreation), then Aquinas would say the "seminal power" of Original Sin is not present; he gives the following analogy to explain this point- "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." Therefore, if a human is conceived, not by any action related to human will, but directly by the will of God, then that human conception (according to Aquinas) could have no Original Sin.

Aquinas' observation above seems consistent with Anselm's thought on this subject (in Anselm's work on Original Sin that I previously examined HERE), but Anselm's discussion strikes me as superior as he gives a tightly reasoned argument from Scripture that relates the creation of Christ in the woman (drawing His human nature from the woman) to the miraculous creation of Adam, and argues that such a direct work of God in conception makes it fitting that the newly created Man would not have Original Sin.