Pohle-Preuss "Transient Act"

“Another effect peculiar to Matrimony is the marriage bond (vinculum matrimoniale), which symbolizes the one and indissoluble union of Christ with His mystic spouse, the Church. This effect strongly resembles the sacramental character imprinted by Baptism, Confirmation, and Holy Orders, and hence is often called quasi-character. Bellarmine and Sanchez regard the marriage bond as a sort of permanent Sacrament. But this view is untenable.

The Sacrament proper (sacramentumtantum) in Matrimony is the transient act by which the conjugal contract is formed, just as the Sacrament of Baptism is the transient act of ablution. But the bond of wedlock is a permanent effect, bearing a striking resemblance to the character imprinted by Baptism, Confirmation, and Holy Orders, and hence must be regarded as res et sacramentum, and may justly be styled "quasi-character," especially in view of the fact that it renders the Sacrament incapable of repletion during the lifetime of both contracting parties. It would be wrong, however, to ascribe to Matrimony a sacramental character in the strict sense. The mark imprinted on the soul by this Sacrament, unlike the character imparted by the other three Sacraments mentioned, is not physical, but purely moral. From the "quasi-character" of Matrimony flow the two properties of Christian marriage, viz.: unity (unitas) and indissolubility (indissolubilitas).”

(Pohle-Preuss, The Sacraments, XI, pp. 170-171)

The "transient act" to which Pohle-Preuss alludes is the consent of the spouses. The betrothed consent to a lifelong contract, so the sacrament has this same character, persisting until death. The conjugal act as such has no contractual or sacramental efficacy, though under previous ecclesiastical law it could be considered evidence of intent to marry.

A marriage not yet consummated has only intrinsic indissolubility. It can still be dissolved by the Pope, for example, as long as the two have not yet become one flesh. In such rare cases, a validly contracted marriage between Christians, and therefore a valid sacrament of matrimony, is being dissolved by authority granted to the Church. [Pope Pius XII discusses this in his 1941 Allocution to the Roman Rota (3rd topic): http://w2.vatican.va/content/pius-xii/it/speeches/1941/documents/hf_p-xii_spe_19411003_roman-rota.html ]