They took vows. The vows precluded marriage. They "married" anyway. That is fornication.
No, your charge of fornication remains dependent on the premise of Rome’s authority as supreme, as it disallows liberty btwn the man and his Maker in regard to personal vows not to marry (etc.) except by her consent.
Not all vows are of God, and if contrary to Scripture (and which presuming that all pastors - some converts excepted - have the gift of permanent celibacy, manifestly is) then these are disallowed, as can be rash vows and vows made before conversion. Otherwise if Herod or the Pharisees of Acts 23:12 were converted they will be still bound to murder, and Buddhist monks would be committed to all theirs.
Under the Law the vow of a women is abrogated if her father or husband disallows it in the period that she made it, (Num. 30:5-8) and when one becomes born again then they have God as their Father, and one would not necessarily be bound to voluntary vows made to Him before that time, such as regards religious devotion (not to eat meat, not to marry, etc.), nor rash vows after that which are contrary to Scripture.
And thus if Katharina’s vow was made in a preconverted state then God could give her liberty to marry afterwards.
Paul himself expressed that as an apostle he had liberty to have a wife as did the other apostles incldg Cephas and the brethren of the Lord, just like he could have only lived off of donations. (1Cor. 9:5,6)
They took vows according to man made beliefs and not what GOD says! GOD reigns and man does NOT!
If fact, God calls those who forbid anyone to marry hypocritical liars - so these liars who forbid marriage fornicated with satan, the master of lies/liars.
"Such teachings come through hypocritical liars, whose consciences have been seared as with a hot iron." 1 Tim 4:2
"They forbid people to marry and order them to abstain from certain foods, which GOD CREATED to be received with thanksgiving by those who believe and who know the TRUTH." 1 Tim 4:3
God's Word is The Final Authority and anyone who doesn't believe that their consciences has been seared as they don't BELIEVE nor KNOW the TRUTH!
There was a time when one could have a somewhat reason-able exchange with you, but that has now passed. Resorting to attacking Luther as if we follow him as a pope, and having Luther and Kathrina driven by sexual so that they snuck out in a fish barrel and whose marriage was no different than homosexual marriage, because you must defend the church they dissented from - which was a church that ordained known sexual active priests (and was rife with such) and is guilty of spiritual idolatry - places you in league with cultists who are bound to do the same due to their sola ecclesia basis.