Objection 1. It would seem that carnal intercourse is an integral part of marriage. For at the very institution of marriage it was declared (Genesis 2:24): "They shall be two in one flesh." Now this is not brought about save by carnal intercourse. Therefore it is an integral part of marriage.
Objection 2. Further, that which belongs to the signification of a sacrament is necessary for the sacrament, as we have stated above (2; 9, 1). Now carnal intercourse belongs to the signification of matrimony, as stated in the text (Sent. iv, D, 26). Therefore it is an integral part of the sacrament.
Objection 3. Further, this sacrament is directed to the preservation of the species. But the species cannot be preserved without carnal intercourse. Therefore it is an integral part of the sacrament.
Objection 4. Further, Matrimony is a sacrament inasmuch as it affords a remedy against concupiscence; according to the Apostle's saying (1 Corinthians 7:9): "It is better to marry than to burn." But it does not afford this remedy to those who have no carnal intercourse. Therefore the same conclusion follows as before.
On the contrary, There was matrimony in Paradise, and yet there was no carnal intercourse. Therefore carnal intercourse is not an integral part of matrimony.
Further, a sacrament by its very name denotes a sanctification. But matrimony is holier without carnal intercourse, according to the text (Sent. D, 26). Therefore carnal intercourse is not necessary for the sacrament.
I answer that, Integrity is twofold. One regards the primal perfection consisting in the very essence of a thing; the other regards the secondary perfection consisting in operation. Since then carnal intercourse is an operation or use of marriage which gives the faculty for that intercourse, it follows, that carnal intercourse belongs to the latter, and not to the former integrity of marriage [Cf. III, 29, 2].
Reply to Objection 1. Adam expressed the integrity of marriage in regard to both perfections, because a thing is known by its operation.
Reply to Objection 2. Signification of the thing contained is necessary for the sacrament. Carnal intercourse belongs not to this signification, but to the thing not contained, as appears from what was said above (1, ad 4,5).
Reply to Objection 3. A thing does not reach its end except by its own act. Wherefore, from the fact that the end of matrimony is not attained without carnal intercourse, it follows that it belongs to the second and not to the first integrity.
Reply to Objection 4. Before carnal intercourse marriage is a remedy by virtue of the grace given therein, although not by virtue of the act, which belongs to the second integrity.
Source: Summa Theologica (New Advent)