Aquinas lived in the 13th century, though, and his formulation out of the natural law didn't occur until the writings of Aristotle were restored to the West. It is Aristotle, after all, who puts such an emphasis on "the nature of the thing" in determining questions about ethics and justice (and indeed even physics). Those writings came to the Church out of Spain, especially following the conquest of Toledo in 1085. When the Christians found themselves in possession of the great libraries of Toledo, rather than burn them (as the Mongols did to the libraries in Baghdad and Persia) they set up teams of translators. Many Christians who spoke and read Arabic lived in the city, as well as Jewish scholars who could read multiple languages. Translation from Arabic into Latin and other languages accessible in Europe became a focus of the Crown of Castille, which provided the funding for the efforts led by the Church.
Before that, marriage did not have the natural law reading in the West. It still had a unique character in Christian civilization, though, opposed to the contractual reading. Marriage was a contract in Islam. Dr. Zorgati explains (p. 102):
According to Charles Donahue, “the most frequently made comparative statement about the Christian law of marriage, on the one hand, and the Islamic [ . . . ] or the Jewish [ . . . ], on the other, is that marriage is a sacrament in Christianity but it is not in Islam or Judaism” (Donahue, 2008, 46). In studies dedicated to Muslim marriages, it is often its contractual nature which is at the forefront. 6 However, the opposition between marriage as contract and marriage as sacrament has to be nuanced. First, there is not one Islamic marriage contract, but many, since different legal schools developed different requirements for the marriage contract, and because people could add individual stipulations to their contracts. Second, although the idea of marriage as a sacrament has roots back to Saint Paul’s letter to the Ephesians, it was first translated into legal doctrine in the twelfth century (Donahue, 2008, 47). According to Islamic law, “marriage is a contract (ʿaqd), established by bilateral agreement” (Ali, 2008, 11). It is a mundane transaction (muʿāmala) which some jurists also saw as an act of worship (ʿibāda) since according to one ḥadīth a married person has fulfilled half of his or her religion (Ali, 2008, 11). Moreover, one of the essential features of the contract is the offer (ījāb) made by the bride’s family and the acceptance (qabūl) of the groom’s family (Ali, 2008, 11– 13). Other important elements are the dower (ṣadāq) and the role played by the guardian and the witnesses, as well as the consent of the contracting parties.The Church's idea ran in contrast to the actual practice of the Christian people: before the 12th century, Christians tended to prefer arranged marriages based on social class and the preservation of the stability of the family. As the Church developed the idea of marriage as a sacrament, though, the sacred character of the bond tended to undermine family authority (Zorgati p. 104):
The insistence of the free consent of the parties must be understood in relation to the developing view that marriage constituted a sacrament. Canonists writing in the decades before Alexander III insisted on the sacramental character of marriage. For example, Peter Lombard established that marriage was one of the seven sacraments of the Church, whereas Hugh of Saint Victor explored the etymology of ‘sacrament’ that he thought corresponded to ‘holy sign’ (sacrum signum). 10 Hence, in addition to the received idea that the relationship between husband and wife was analogous to the relationship between Christ and the Church— a mystery, or sacrament, according to Saint Paul— he saw marriage as a sign of the mutual love between the soul and God. This new idea had, according to Donahue, an impact on the doctrine of free consent in marriage which developed at the same time: “A theology that sees in marriage a sign of the mutual yearning of the soul for God and of God for the soul would tend to emphasize, as Hugh does, the element of choice in marriage, and would tend to exclude the choice of anyone else as being relevant to the question of the formation of marriage” (Donahue, 2008, 54).That's an interesting view, and one that is in contrast with the view that Aquinas came to in the next century. The principal end of matrimony in that view, derived from "the nature of the thing," is filling the need for humanity to reproduce itself across generations: not only to procreate, but to educate and develop children so they are able to sustain themselves and support the greater society of which they are part.
Unlike the Islamic and Jewish contractual view, the Christian view permitted the two parties who loved each other to come together regardless of their rank in society, but only by their own free choice. Also unlike the contractual views, however, divorce was forbidden. The Love that could unify a man and a woman of different ranks into one flesh was a miracle. None should dare to live in defiance of such a miracle.