A formal definition by a council is not always the first instance something was accepted as true (or as a sacrament). Often the council reacted to an attack on an already accepted tenet of faith. Because the attack, the position of the Church had to be officially defined, in order to be formally defended.
There are patristic sources, among whom Augustine, who call Marriage a sacrament, and others who early on describe it in sacramental terms.
"And these are the nuptials of the Lord, so that like that great Sacrament they might become two in one flesh, Christ and the Church. From these nuptials a Christian people is born, when the Spirit of the Lord comes upon that people."
Pacian, Sermon on Baptism,6(ante A.D. 392),in JUR,II:144
"It is certainly not fecundity only, the fruit of which consists of offspring, nor chastity only, whose bond is fidelity, but also a certain sacramental bond in marriage which is recommended to believers in wedlock. Accordingly it is enjoined by the apostle: 'Husbands, love your wives, even as Christ also loved the Church.' Of this bond the substance undoubtedly is this, that the man and the woman who are joined together in matrimony should remain inseparable as long as they live..."
Augustine, On Marriage and Concupiscence,1,10(A.D. 420),in NPNF1,V:268
Within the Catholic Church, the International Theological Commission wrote the document 'Propositions on the Doctrine of Christian Marriage'. In it, they write the following, confirming the comments by Matthew on the main question that further clarify that the Church sees this sacrament as going back to Christ:
2.1. Real Symbol and Sacramental Sign
Jesus Christ disclosed in a prophetic way the reality of matrimony as it was intended by God at man’s beginnings (cf. Gen 1:27; 2:24; Mk 10:6, 7-8; Mt 19:4, 5) and restored it through his death and Resurrection. For this reason Christian marriage is lived “in the Lord” (1 Cor 7:39) and is also determined by elements of the saving action performed by Christ.
Already in the Old Testament the matrimonial union was a figure of the Covenant between God and the people of Israel (cf. Hos 2; Jer 3:6-13; Ezek 16 and 23; Is 54), In the New Testament, Christian marriage rises to a new dignity as a representation of the mystery that unites Christ and the Church (cf. Eph 5:21-33). Theological interpretation illuminates this analogy more profoundly: the supreme love and gift of the Lord who shed his blood and the faithful and irrevocable attachment of his Spouse the Church become models and examples for Christian matrimony.
This resemblance is a relationship of real sharing in the Covenant of love between Christ and the Church. From its own standpoint, Christian marriage, as a real symbol and sacramental sign, represents the Church of Christ concretely in the world and, especially under its family aspect, it is called rightly the “domestic Church” (LG 11).
2.2. Sacrament in a Real Sense
In such a way matrimony takes on the likeness of the mystery of the union between Jesus Christ and his Church. This inclusion of Christian marriage in the economy of salvation is enough to justify the title “sacrament” in a broad sense.
But it is also at once the concrete condensation and the real actualization of this primordial sacrament. It follows from this that Christian marriage is in itself a real and true sign of salvation, which confers the grace of God. For this reason the Catholic Church numbers it among the seven sacraments (cf. DS 1327, 1801).
A unique bond exists between the indissolubility of marriage and its sacramentality, that is, a reciprocal, constitutive relationship. Indissolubility makes one s grasp of the sacramental nature of Christian matrimony easier, and from the theological point of view, its sacramental nature constitutes the final grounds, although not the only grounds, for its indissolubility.