De acordo com este site decisão foi tomada por Alan Burnett, Bruce Timm e Brian Azzarello juntos:
The decision for the two superheroes have casual sex, made by Bruce Timm, Alan Burnett, and screenwriter Brian Azzarello, changes several aspects of both the character’s histories, and twists the film’s overall interpretation of Alan Moore’s infamous comic. Although a sexual or romantic relationship between the two had been teased in some ancillary Batman canon projects (specifically in Batman Beyond), their union is not a mainstay in Gotham stories. Still, director Bruce Timm defended the decision to put in a Wayne-Gordon sex scene.
“It was really important to us to show that both of the characters make some pretty big mistakes. I mean, his ‘parental skills’ aren’t that great,” Timm told Vulture. “Maybe never having had any kids of his own, he doesn’t realize that if you tell a kid to not do something, they’re going to want to do it even more. And then she makes some mistakes and then he kind of overreacts to her mistakes and then she overreacts to his overreaction. So it’s very human; it’s a very understandable story.”
Na entrevista , o site links para Timm diz isso especificamente, embora tentativamente coloque a ideia inicial aos pés de Brian Azzerello:
I'm guessing there will be a lot of conversation after the movie's release about the sex scene. At what point in the creative process did you come up with that idea?
It came from a three-way conversation between [co-producer] Alan Burnett, [screenwriter] Brian Azzarello, and myself. I don't remember who initially came up with the idea, but we all kind of jumped on it all at the same time and said, Yeah, that's kind of where we need to go. My memory kind of says it was Brian, maybe, who came up with the idea. But I'm not sure.
Naturalmente, deve-se notar que Azzarello não trabalhou nas encarnações anteriores do Batman animado com Bruce Timm e Alan Burnett, como Batman Beyond (onde ambos co-desenvolveu e produziu a série), que incluiu como backstory que Bruce e Barbara uma vez tiveram um relacionamento, e Batman: Mistério da Batwoman (escrito por Burnett), que também tinha algumas indicações de um relacionamento, ou pelo menos atração. Então, embora Azzarello possa ter sido o primeiro a dizer isso na mesa, também é possível que ele tenha tido a ideia dos outros dois ali e do seu trabalho anterior, e meramente foi o único a expressar a ideia que eles deveriam colocá-lo na tela neste filme em particular.