It may be wrong to paint the picture in black and white if the professor has indeed done that. Through the centuries, things were literally all over the map. There was chronological and geographical diversity.
The development of what constituted marriage, consent or consummation, had a long period of development in theology prior to the middle ages. There was flux over time and different practices or laws from one region to another. That was also true in terms of ecclesiastical ceremony and its relationship to what a man and woman actually did.
The imposition of a standard canonical form of marriage or even requiring the assistance of a priest (to prevent clandestine marriage and to guarantee the freedom of the parties from impediments) whether for validity or legality was a later development, and it took a long time before it became universally binding and effective. Despite Lateran IV and Trent, that issue really didn’t settle down until Ne Temere in 1907 (ewtn.com/library/CURIA/NETEMERE.HTM)).
To establish marriage in the first Christian millenium, it was considered sufficient by the Church for the couple to declare intent in some way and to set about the business of bringing children into the world. They were considered married by the Church and were not considered free by the Church to abandon the union or to marry another person. There was no prescribed form, and the Church generally followed tribal or regional custom. Those customs varied widely. The experience in England was not necessarily that of the Teutons or Franks in continental Europe nor that among Christians in Africa or Asia.
Given that the presence of a priest might be very infrequent in many parts of the world where Christians lived, people basically followed what we’d call the extraordinary form of marriage today — consent of some kind and witnesses. So if they consented, consummated, cohabited and procreated, they were considered married by the Church. That much is well established.
On one hand, there’s early evidence of ecclesiastical ceremony, but little indication as to how it might have related to betrothal and the manifestation of consent. Was it subsequent to that, or concurrent? Was a declaration of betrothal itself sufficient to manifest consent? There is some evidence, again, of variation.
But I would suggest looking as early as the ante-Nicean Fathers. Visit Tertullian (died about 220): (tertullian.org/anf/anf04/anf04-13.htm#P912_220501) for a start (Ad Uxorem Libri Duo). He is at least provocative when we consider his language carefully. Here’s an English translation from that site though I think the Latin might be more telling.
“Whence are we to find (words) enough fully to tell the happiness of that marriage which the Church cements, and the oblation confirms, and the benediction signs and seals; (which) angels carry back the news of (to heaven), (which) the Father holds for ratified?” (“Unde sufficiamus ad enarrandam felicitatem eius matrimonii, quod ecclesia conciliat et confirmat oblatio et obsignat benedictio, angeli renuntiant, pater rato habet? Nam nec in terris filii sine consensu patrum rite et iure nubunt.”)
I understand that provision was made for a ceremony coram ecclesiam (in the presence of the Church) in the Leonine, Gelasian, and Gregorian sacramentaries (Leo [440-61], Gelasius [492-6], and Gregory the Great [590-604]). You may want to pursue that although it doesn’t address the question of a couple expressing consent and consummating before hoofing it to the local parish.
On the other hand, in some jurisdictions, there was an early tradition requiring ecclesiastical ceremony to constitute a valid and lawful marriage. You might take a quick peek at the NewAdvent.org and the Catholic Encyclopedia, newadvent.org/cathen/09703b.htm.