Section — Prümmer, Manuale Theologiae Moralis
§ 2. On Legal Relationship
783. Notion. Legal relationship is the proximity of persons arising from civil legal adoption.
This impediment is diriment between those persons who by the civil law of their country are declared incapable of nuptials because of legal adoption; but it is merely impedient if the same civil law declares such nuptials only illicit. Thus the Code of Canon Law, cann. 1059 and 1080, establishes. Therefore ecclesiastical discipline no longer canonizes and adopts the ancient Roman law in this matter, but the civil laws of each region. Hence the ecclesiastical impediment of legal relationship also seems to be totally abolished in those regions (for example, in Austria, Holland, England, Chile, Mexico, and so forth) where civil law neither invalidates nor prohibits matrimony between the adopted person and the adopter.16
St. Thomas17 explains the fittingness of this impediment most excellently thus: "Divine law especially excludes from matrimony those persons whom it was necessary to live together, lest, if it were lawful for them to use carnal union, an easy occasion for concupiscence should lie open; matrimony is ordained to repress this. And because an adopted son dwells in the house of the adopting father as a natural son, therefore by human laws it is prohibited that matrimony be contracted between such persons, and such a prohibition is approved by the Church."
Origin of this impediment. Legal relationship as a diriment impediment of matrimony was first established by ancient Roman law.18 This civil law was afterward canonized by the Church. But when this was done is not so certain. Gratian already presupposes the existence of the impediment of legal relationship;19 but Raymond in the compilation of the Decretals placed a special title concerning legal relationship,20 which he afterward briefly and clearly circumscribed in his Summa.21
Extent of the impediment. Legal relationship dirimes or prohibits matrimony according to the civil laws of each region. In Germany, France, Hungary, Switzerland, Belgium, and Luxembourg, legal relationship is a more or less impedient impediment; in Italy, Spain, Poland, Brazil, and so forth, this relationship is a diriment impediment.
Cessation of this impediment. Since legal relationship, according to present ecclesiastical discipline, is judged entirely according to the civil laws of each region, it also seems to cease in the ecclesiastical forum when it has ceased in the civil forum. Whether the Holy See will concede a dispensation from this impediment in a case of necessity if civil law invalidates or prohibits such a matrimony is not yet established.
16 Legal relationship seems to be a diriment impediment in Spain and Italy, but not in Germany, Switzerland, France, Austria, Holland, or Belgium.
18 Instit. Iustin., l. 1, tit. 11; Pandect., l. 1, tit. 7; Cod., l. 8, tit. 48; Novell. 74, c. 3, and 89.
19 c. 1 § 6, C. 30, q. 3.
20 L. 4, tit. 12.
21 Summa, l. 4, tit. 8.