Luther and his consort were not free to marry therefore it was fornication and not a proper marriage.
Says who?
1Ti 4:1 ¶ Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
1Ti 4:2 Speaking lies in hypocrisy; having their conscience seared with a hot iron;
1Ti 4:3 Forbidding to marry, [and commanding] to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth.
1Ti 4:4 For every creature of God [is] good, and nothing to be refused, if it be received with thanksgiving:
1Ti 4:5 For it is sanctified by the word of God and prayer.