In the introduction to Unmarriages: Women, Men, and Sexual Unions in the Middle Ages, Ruth Mazo Karras goes over conceptions of marriage according to the Bible, late antique Rome, and evolving ideas in the medieval Christian Europe. She even talks about myths and realities of "Germanic marriage." Karras also has the virtue of being much more readable and understandable than other authors covering some of the same territory.

One really interesting question that arises in medieval Latin Europe is: what makes a marriage a valid marriage. Is it the sacrament performed by a priest, the consent of the two parties, or the act of consummation? Canon law actually comes down on the side of consent. The arguments for the three views are definitely steeped in medieval thought, but you might find something of interest to translate into your own worldview there. James Brundage's Law, Sex, and Christian Society in Medieval Europe has a good discussion of that particular development (but you'd probably want to bypass most of the rest of the book, since it's really about the development of canon law using marriage as its running example).