When one has yet to "come out," he or she is not yet officially gay. Thus, he/she is brokeback to the future. In the future (when he/she comes out of the closet), he/she will be fully and openly gay.
Inspired by the internet parody trailer "Brokeback to the Future."
Dude 2: Yeah man. He's fuckin' brokeback to the future.
Dude 3: Why the hell were you guys watching Little Mermaid together last night? I think all of you are brokeback to the future.