Steve and Bucky's indomitable friendship was the beating heartbeat of "Captain America: The Winter Soldier," arguably Marvel's best movie to date. Since then, fans have been shipping Bucky Barnes (Sebastian Stan) as Steve Rogers' (Chris Evans) one true love in the superhero franchise. (Sorry, Peggy.)

It's not for lack of subtext. When the action-packed first look at the third and final "Captain America" film, "Captain America: Civil War," debuted at Disney's D23 Expo in August, Tumblr exploded. Amid all of the action and excitement, this bit of dialogue made even the fairweather-ist of slash fans' hearts flutter: "He remembered you. Your pal, your buddy, your Bucky."

So, why do audiences cling so dearly to the relationship between Cap and his one-time best friend turned mortal enemy turned friend again? It's not uncommon for fans to ship same-sex couplings, often referred to as "slash" pairings. Mere hours after the release of "Star Wars: The Force Awakens," the Internet was buzzing over Poe Dameron and Finn. Not to mention, the Internet loves to couple up the Science Bros. -- aka Bruce Banner and Tony Stark from "The Avengers."

While there can sometimes be a disconnect between the creators that birthed the fandom and the fandom itself, "Captain America" co-director Joe Russo is totally cool with people shipping Stucky, romantically or not.

"People can interpret the relationship however they want to interpret it," Russo said while promoting their new film studio in China. "For us, we've always interpreted the relationship as two brothers. They're very close characters, they have a relationship with each other that is very deep. The bonds between the characters are very strong. That's what motivates the storytelling."

"These are both characters that came from nothing," he added. "Captain America was basically an orphan, and Bucky’s family took him in... When he was sleep for several years, he lost everything that was dear to him. And when he took the serum and became Captain America, he gave away a large part of himself for a patriotic cause. So, you have a character who is searching for the only thing that he has left from his past... and that’s Bucky."

Although Russo sees Steve and Bucky's relationship as more fraternal than romantic, that doesn't mean he doesn't support reading into the homoerotic subtext of their friendship.

"People have interpreted that relationship all kinds of ways, and it's great to see people argue about it what that relationship means to them," he said. "We will never define it as filmmakers, explicitly, but however people want to interpret it."