I've just finished reading Lyndal Roper's "Martin Luther: Renegade and Prophet." I knew little about Luther before I read the book, other than Western Civ. in high school and the Netflix documentary starring Hugh Bonneville.
A couple of things that I learned: any woman giving birth "was under the sway of the devil, and that if she were to die before being churched, she could not be buried in the church yard." I think the Roman Catholic Church outdid itself on that one!
The secular powers had little say in the institution of marriage: "As a sacrament, it had fallen under the purview of the Church, which decided which marriages were permitted and what counted as incestuous and required dispensations; it all was made more complex because godparenthood created an additional spiritual network of kinship and so a host of potentially incestuous unions." So this was where the Church was when Luther wrote "On the Babylonian Captivity of the Church," where he began evolving his ideas about marriage, eventually denying that it was a sacrament.
Roper also devotes the final chapter of her book to Luther's anti-Semitism. To read his words about what he believed Christians should do to the Jews is horrifying. Anti-Semitism was essential to Luther's theology: "it was not incidental to his theology, a lamentable prejudice taken from contemporary attitudes. Rather, it was integral to his thought; his insistence that the true Christians- that is, the evangelicals- had become the chosen people and had displaced the Jews would become fundamental to Protestant identity."