When you sleep with your wife Catherine and embrace her, you should think: ‘This child of man, this wonderful creature of God has been given to me by my Christ. May he be praised and glorified.’ On the evening of the day on which, according to my calculations, you will receive this, I shall make love to my Catherine [von Bora] while you make love to yours, and thus we will be united in love.

 
One of Luther’s most significant gains for the evangelical cause was his championing of the virtues of marriage and of sex. “Whoever is ashamed of marriage”, says Luther, “is ashamed of being human”. Suggestions that marriage, and therefore, sex are somehow unspiritual is a slander and an attack of the Devil. Luther discarded the medieval notion that reason is “high” and the physical is “low” or base. The sex drive was, for Luther, a divine force, part of God’s vital presence in man since man’s physicality is part of his being made in the image of God. Faith, for Luther, is lived out in a physical experience and, thus, in his Larger Catechism, he explains the first line of the Creed thus:

What I mean and believe is that I am God’s creature, that means that he has given me and continuously maintains body, soul and life, limbs small and large, all the senses, intellect and reason.

 
No area of life should be thought of as unspiritual or unholy. Sexual desire and fulfilment are healthy and God-given. Thus, in his Against the so-called Spiritual Estate (1522) Luther explains

A young woman, if the high and rare grace of virginity has not been bestowed upon her, can do without a man as little as without food, drink, sleep and other natural needs. And on the other hand, a man, too, cannot be without a woman. The reason is the following; begetting children is as deeply rooted in nature as eating and drinking. That is why God provided the body with limbs, arteries, ejaculation, and everything that goes with them. Now if someone wants to stop this and not permit what nature wants and must do, what is he doing but preventing nature from being nature, fire from burning, water from being wet, and man from either drinking, eating or sleeping.

 
In the Reformation Luther redeemed for us much of what had been lost in medieval Christianity. This was not just true theologically but practically also. He restored dignity to manual labour and the world of work, rejecting the notion that the “clergy” (what a horrible and unbiblical word!) are somehow more spiritual than the laity. His Address to the Christian Nobility of the German Nation (1520) still stands as one of the foundational building blocks of evangelical thought today in re-establishing the Biblical principle of the priesthood of all believers. As far as marriage and sex are concerned, he took them from the “unholy” and the “dark hours of the night” and elevated them to a whole new level. He led by example, of course, marrying Catherine von Bora in 1525 “in defiance of the Devil” (probably not the first reason any of us would give for marrying). A former monk marrying a former nun was, at the time truly scandalous. The birth of their first child was awaited with a degree of fear and trepidation as popular superstition held that a two headed monster would be the product of such a sacrilegious union. Within a few years, however, Luther was boasting “I have legitimate children which no papal theologian has!” Let’s be people who, like Luther, celebrate work, marriage, sex, family life and everything else God has provided. As physical beings God has given us physical things for our enjoyment.
 
For more on this see HA Oberman, Luther: Man between God and the Devil (1989), chapter 10 “Wedded Bliss and World peace: in Defiance of the Devil”