“‘Let the marriage be held in honor among all, and let the marriage bed be undefiled’ (Hebrews 13:4) Hold fast to that, those of you who are married. St. Augustine writes in one place concerning married people, that even if one of them is somewhat weak, etc., he should not be afraid of the sudden and infallible Day of the Lord; even if the day of the Lord were to come in the hour when man and wife were having marital intercourse, they should not be afraid of it. Why is this so? Because even if the Lord comes in that hour He will find them in the ordinance and station in which they have been placed and installed by God.”
From Luther’s Sermon for the Marriage of Sigismund von Lindenau, 1545, LW 51
Concordia and Koinonia 