Saint Gall, Stiftsbibliothek 673, p. 170

<GRATIANUS> Set quod sit inter eos sit coniugium sic probatur.  Coniugium uel matrimonium est uiri et mulieris coniunctio indiuiduam uite consuetudinem continens. Item consensus utriusque matrimonium facit.   Quoniam ergo ab hoc coniuncti sunt ut indiuiduam uite consuetudinem conseruarent, quia uterque in alterum consensit, coniuges sunt appellandi. (That there was a valid marriage is thus proven:  Marriage is the union of a man and woman keeping the custom of an undivided way of life [Instit. 1.9.1.]. The consent of both makes a marriage. Those, therefore, who join in order to keep an undivided way of life, each consenting in the other, must be called married).

Justinian's  Institutes 1.9.1:  Nuptiae autem sive matrimonium est viri et mulieris coniunctio individuam consuetudinem vitae continens.

Justinian, Digest  35.1.15 Ulpianus 35 ad sab. Nuptias enim non concubitus, sed consensus facit.

Justinian, Digest 50.17.30: Ulpianus 36 ad sab. Nuptias non concubitus, sed consensus facit.