By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state. It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called "sin" only in an analogical sense: it is a sin "contracted" and not "committed" - a state and not an act.

Catechism of the Catholic Church ... III Original Sin ... Paragraph 404

The catechism here makes clear that sin is a matter of the transmissible human nature. Sin is a matter of humanity, of flesh and blood. And the flesh and blood, the humanity, that is inherited by natural generation, is that in which sin is transmitted.

By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state. It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice [sic]. And that is why original sin is called "sin" only in an analogical sense: it is a sin "contracted" and not "committed" - a state and not an act.

Catechism of the Catholic Church ... III Original Sin ... Paragraph 404

The catechism here makes clear that sin is a matter of the transmissible human nature. Sin is a matter of humanity, of flesh and blood. And the flesh and blood, the humanity, that is inherited by natural generation, is that in which sin is transmitted.