How did Karl Barth justify his relationship with Charlotte von Kirschbaum? I recently learned that Karl Barth, while he was married, had a decades-long romantic relationship with his personal assistant, Charlotte von Kirschbaum.  She actually moved into the Barths' home in 1929, straining the family.

I'd like to know how Karl Barth justified this relationship.  After a few internet searches and my own recollections of Barth's theology, the following possibilities come to mind:


Barth did not have a sexual relationship with von Kirschbaum, and therefore did not think that he was committing adultery
Barth, who argued that the Bible "contains" the Word of God, did not consider the prohibition of adultery in the Ten Commandments as "the Word of God" and therefore not binding
Barth could not overcome his love for von Kirschbaum, and thus considered it acceptable.


Which of these arguments, or others that I might be missing, did Barth use?  How did Barth justify his relationship with von Kirschbaum?
A 2017 Christianity Today article about this very issue was written by Mark Galli shortly after his own Karl Barth biography was published.  As curiousdannii suspected, he did use dialectical tension to justify it, part of the discussion by Christiane Tietz in Theology Today, of the personal letters Barth wrote to von Kirschbaum from 1925–1935.

A lot more details, analysis, and paraphrases from Tiez's article can be found in a very well-written blog article here.  The blog article in turn highly recommends a "book review" of Charlotte von Kirschbaum and Karl Barth, 1998 which actually not so much a book review since 85% of it is about relevant external history of their relationship.