It all makes sense.
Bert and Ernie have been a mainstay on Sesame Street since 1969, which means the popular puppets have influenced several generations of children. In 1984, Mark Saltzman joined the Sesame Street crew as a writer, and began to work on scenes for several of the show's main characters. Bert and Ernie fell under Saltzman's creative umbrella, and he worked with the legendary characters for years. In an interview with Queerty, Saltzman revealed that he fashioned Bert and Ernie after his own gay relationship.
"I remember one time that a column from The San Francisco Chronicle, a preschooler in the city turned to mom and asked 'are Bert & Ernie lovers?'" stated Saltzman. "And that, coming from a preschooler was fun. And that got passed around, and everyone had their chuckle and went back to it. And I always felt that without a huge agenda, when I was writing Bert & Ernie, they were."
He continued on to highlight his own relationship with late film editor Arnold Glassman. "I didn’t have any other way to contextualize them. The other thing was, more than one person referred to Arnie & I as 'Bert & Ernie.' I was Ernie. I look more Bert-ish. And Arnie as a film editor—if you thought of Bert with a job in the world, wouldn’t that be perfect? Bert with his paper clips and organization? And I was the jokester. So it was the Bert & Ernie relationship, and I was already with Arnie when I came to Sesame Street. So I don’t think I’d know how else to write them, but as a loving couple. I wrote sketches…Arnie’s OCD would create friction with how chaotic I was. And that’s the Bert & Ernie dynamic."