Thomas Aquinas

“ On the contrary, Relationship is an impediment to marriage. Now a kind of relationship results from adoption, namely legal relationship, as evidenced by its definition, for "legal relationship is a connection arising out of adoption." Therefore adoption results in a tie which is an impediment to marriage. ”

“ One may lose one's grandchildren and so forth even as one may lose one's children. Wherefore since adoption was introduced as a solace for children lost, just as someone may be adopted in place of a child, so may someone be adopted in place of a grandchild and so on. ”

“ Wherefore anyone can indifferently become another's godfather. But adoption aims at hereditary succession and a certain subjection of the adopted to the adopter: and it is not fitting that older persons should be subjected to younger in the care of the household. Consequently a younger person cannot adopt an older; but according to law the adopted person must be so much younger than the adopter, that he might have been the child of his natural begetting. ”

“ Further, the relationship that impedes marriage is a perpetual impediment. But there is not a perpetual impediment between the adopted son and the naturally begotten daughter of the adopted; because when the adoption terminates at the death of the adopter, or when the adopted comes of age, the latter can marry her. ”

“ Even a woman can adopt by permission of the sovereign, wherefore legal relationship passes also to her. Moreover the reason why spiritual relationship does not pass to a non-baptized person is not because such a person cannot be a god-parent but because he is not a fit subject of spirituality. ”

“ But no tie of relationship is contracted through one's being subjected to another's spiritual care: else all those who dwell in the parish would be related to the parish priest and would be unable to marry his son. ”

“ I answer that, The Divine law especially forbids marriage between those persons who have to live together lest, as Rabbi Moses observes (Doc. Perp. iii, 49) , if it were lawful for them to have carnal intercourse, there should be more room for concupiscence to the repression of which marriage is directed. ”

“ In this way even one who is not his own master can adopt, without the consent of the sovereign and with the authority of a magistrate: and one who is thus adopted does not inherit the estate of the adopter, nor is the latter bound to bequeath to him any of his goods in his will, unless he will.
This suffices for the Reply to the Second Objection.
Reply to Objection 3: Natural procreation is directed to the production of the species; wherefore anyone in whom the specific nature is not hindered is competent to be able to beget naturally. ”