Martin Luther married Katharina von Bora (1499-1552) on June 13, 1525. Katharina came from a family of higher social rank than Luther’s family. The Von Boras were members of the knightly class, a proud but declining segment of German society. Luther came from peasant stock. His father Hans was a miner who got involved in the business side of the mining and became well-to-do. Katharina was only three when she was sent away to school and eventually took vows to become a nun.
Was their marriage based on love for each other? you ask. We can only speculate due to various comments Luther is known to have written, as hardly anything can be found on Katharina from herself. I have not seen the documentary you mention. The following I have extracted from chapter 17 (The School For Character) in the book Here I Stand by Roland Bainton (Lion, 1978) and the historical novel by Margaret Skea, Katharina: Deliverance - A novel of the wife of Martin Luther (Sanderling Books, 2017).
Luther had no thought of marrying but when monks began to marry during his stay at the Wartburg [from 4 May 1521] he had exclaimed,
"Good heavens! They won't give me a wife." After the event he said
that if anyone had told him at Worms that in six years he would have a
wife, he would not have believed him.
The problem with monks and nuns was so bad, Luther helped many nuns escape. One event had been prompted by some Cistercian sisters in a nearby village seeking Luther’s counsel as to what they should do in view of their evangelical persuasion. He took it upon himself to arrange their escape. This plot was carried out on the Eve of the Resurrection in April 1523 when 60-year-old Burgher of Torgau, Leonard Kopp, put 12 nuns into his covered wagon full of empty barrels of herring from a convent.
Luther felt responsible to find them homes, husbands, or positions.
When someone suggested he marry one of them, he wrote on November 30,
1524 that he had no such intention, not because he was a sexless
stone, nor because he was hostile to marriage, but because he expected
daily the death of a heretic.
Well, two years after Katherina's escape she was still in domestic service. She was intended for a young patrician of Nurnberg but his family objected and he married someone else. So, Luther selected a Dr. Glatz but she would not accept him. In those days, a woman of 26 was viewed as at the upper limits of eligibility for marriage. She asked a Dr Amsdorf of Magdeburg to tell Luther that she could not abide Glatz but she would take Amsdorf himself, or Luther.
He did not respond seriously to the suggestion until he went home to
visit his parents. His father said it was a realistic proposal. In May
1525 he decided to marry Katie. It was no love match, though. He gave
three reasons for his marriage - to please his father; to spite the
pope and the Devil; and to seal his witness before martyrdom. On 27
June 1525 they were married. His invitation to Spalatin read, "You
must come to my wedding. I have made the angels laugh and the devils
Thus a 42-year-old former monk and a 26-year-old former nun voluntarily entered into holy matrimony on June 13, 1525.
On home and his wife, Luther is reputed to have said in 1534:
“At home I have good wine and beer and a beautiful wife, or (shall I
Luther liked to tease Katherine and she gave him back as good as she got. During one dinner table conversation, Luther remarked,
“The time will come when a man will take more than one wife.”
Katherine responded, “Let the devil believe that!” to which Luther
answered, “The reason, Katy, is that a woman can bear a child only
once a year while her husband can beget many.” Undismayed, Katherine
cited from First Corinthians 7:2. “Paul said that each man should have
his own wife.” Luther quipped back, “Yes, ‘his own wife’ and not ‘only
one wife,’ for the latter isn’t what Paul wrote.” The jesting
continued for a while longer until Katherine ended the discussion when
she said, “Before I put up with this [polygamy], I’d rather go back to
the convent and leave you and all our children.”
I suggest that when all the available accounts are put together, we get a balanced picture which does not require an "either, or" conclusion. It seems to me that they grew in mutual love and respect, without losing any of their individuality and characters. But I doubt very much if they were “in love” with each other prior to their wedding. It might be worth mentioning that the Bible commands Christian husbands to so love their wives as Christ loved the church he died for (before the still-future marriage of Christ to his bride), and wives are to respect their husbands. The key point seems to me to be that husbands must do what they find most difficult to do - show considerate love - while wives must do what they find most difficult to do - give respect. Nowhere in scripture are married couples exhorted to "be in love" with each other prior to marrying. Yet love is a key requirement in matrimony, not least because it engenders respect.