You will have to provide a citation for your dogmatic assertion that the Catholic Church teaches that original sin is passed strictly in the paternal line. “Natural propagation” is what the Church defined at the Council of Trent.
The Apostle says (Rm. 5:12): "By one man sin entered into this world, and by sin death." Nor can this be understood as denoting imitation or suggestion, since it is written (Wis. 2:24): "By the envy of the devil, death came into this world." It follows therefore that through origin from the first man sin entered into the world.
I answer that, According to the Catholic Faith we are bound to hold that the first sin of the first man is transmitted to his descendants, by way of origin. For this reason children are taken to be baptized soon after their birth, to show that they have to be washed from some uncleanness.
Reply to Objection 1. The son is said not to bear the iniquity of his father, because he is not punished for his father's sin, unless he share in his guilt. It is thus in the case before us: because guilt is transmitted by the way of origin from father to son, even as actual sin is transmitted through being imitated.
Reply to Objection 2. Although the soul is not transmitted, because the power in the semen is not able to cause the rational soul, nevertheless the motion of the semen is a disposition to the transmission of the rational soul: so that the semen by its own power transmits the human nature from parent to child, and with that nature, the stain which infects it: for he that is born is associated with his first parent in his guilt, through the fact that he inherits his nature from him by a kind of movement which is that of generation.
Reply to Objection 3. Although the guilt is not actually in the semen, yet human nature is there virtually accompanied by that guilt.
Reply to Objection 4. The semen is the principle of generation, which is an act proper to nature, by helping it to propagate itself. Hence the soul is more infected by the semen, than by the flesh which is already perfect, and already affixed to a certain person.
Also, in his Treatise On Evil, Aquinas writes on QQ. 4, A. 7, response 6, that "Christ was unable to contract original sin because he was conceived of the Virgin without semen from a man"
The idea that Christ had no earthly father because Adam also did not seems flawed, since he had no mother either. I can't find where that explanation is an official Catholic doctrine either.
The most literal interpretation of Romans 5:12 - "sin entered the world through one man" - would mean that original sin is transmitted through the father. Aquinas obviously understood it to mean that.