Luther had some pretty crazy views about sex and marriage. For him, marriage is basically a medicine for the animal lust that is the human sex drive. The desire for sex is simply always sinful and marriage is given as a way to satiate that sinful desire in a somehow not sinful way.
This had some rather wild implications. On at least one occasion, Luther castigated a man who married a woman without revealing beforehand his impotence. Luther viewed the man’s deception as completely and utterly outrageous. Since he wasn’t able to fulfill his husbandly duty, in Luther’s mind he was not a real husband at all. Thus, Luther advised the woman to simply start a sexual relationship with someone else (though he did think she should ask her legal husband’s permission first). But he also said that if her husband wouldn’t allow it, then she should simply bolt in the night and find a new husband somewhere else (See WA 6, 558–59).
However, Luther was equally hard on the women who didn’t want to give out as much sex as their husband’s felt like (Luther advised sex at least 2-3 times per week in marriage, by the way). If a wife isn’t open to fulfilling her husbands needs, then according to Luther, “. . . it is time to say: ‘If you don’t want, then another one will. If the wife does not want sex, then let the maid come.’ However, this should only be said when the husband has warned her wife two or three times, and thereafter he has informed other about her stubbornness . . . if even then the wife is unwilling, then let her go and let an Esther be given to you, and let the Vasthi go, just as King Ahasuerus did” (WA 10/II, 290, 8–14).
There’s some marital advice for you!
I encountered this material through a book I recently copyedited, Engaging Luther: New Perspectives, edited by Olli-Pekka Vainio, forthcoming from Cascade Books. It is a very important collection of essays for those interested in Luther and particularly the direction of the “Finnish School” of Luther interpretation. It constitutes the first volume put out by Finnish scholars that examines the breadth of Martin Luther’s theology.