Fantastic Beasts and Where To Find Them has been a controversial follow-up series to the Harry Potter franchise since the premiere. Initially billed as a trilogy focused on a gentle Hufflepuff and his animals, the story turned out to be a prelude to the showdown between Gellert Grindelwald and Albus Dumbledore, a story Rowling has revealed in fits and spurts to have been the latter's first great love. However, none of those details have made it on screen or in the books, which is why J.K. Rowling's quotes about Dumbledore and Grindelwald's relationship feel slightly incongruous with the reality of the franchise.
Rowling's has become well known for adding in details about her characters after the fact. Her first revelation after the Potter books ended was that Dumbledore was supposed to be gay. Though she didn't go into detail at the time, she suggested the story between Dumbledore and Grindelwald in their teenage years was a "love that dare not speak its name" type trope. It wasn't even clear if it was just Dumbledore having a crush on the young, handsome wizard and Grindelwald manipulating him, or if they both felt this way.
At the time, Rowling thought herself done with the Wizarding World, which is why she could drop such details and never worry about having to show them on screen. Now that the new Fantastic Beasts story now revolves around the fallout from that very relationship, Rowling has had to get more explicit.
The latest film, Fantastic Beasts: The Crimes of Grindelwald, did not show any flashback to the teenage relationship between Dumbledore and Grindelwald. The two characters never even appeared on screen together. But as part of the Blu-ray extras, Rowling decided to go full slashfic on unsuspecting viewers and tell them what's not shown.
Their relationship was incredibly intense. It was passionate, and it was a love relationship. But as happens in any relationship, gay or straight or whatever label we want to put on it, one never knows really what the other person is feeling. You can’t know, you can believe you know. So I’m less interested in the sexual side – though I believe there is a sexual dimension to this relationship – than I am in the sense of the emotions they felt for each other, which ultimately is the most fascinating thing about all human relationship.
Rowling has never been great at the sexual side of any of her relationships. Both Harry and Ginny and Ron and Hermione never share more than a kiss on the page or in the movies/on stage. Also, she has both teenage romances become lifelong marriages, so the idea Dumbledore and Grindelwald are still not over each other decades later is at least in keeping with her simplistic "teenage relationships are forever" view of the Wizarding World.
It's this fact which has made the Potter universe such a rich and fertile ground for fanfiction (and slash fiction) over the years. So while fans may be frustrated by the lack of any Dumbledore/Grindelwald action on screen, Potter fic writers can keep doing what you're doing. It's most likely the only action Fantastic Beasts fans will ever see in this department.