So no evil villian, what is the hero fighting against? Stories need the protagonist and antagonist, they don't need the love interest. I don't recall love interest being part of it unless you reference one of the
7 original stories in which love is only part one I can think of off hand. Pretty sure you're conflating
Campbell with
Three act structure which clearly doesn't not state "hero must have love interest" only "everyone knows better who they are" by the end of the story.
Typically, superhero movies fall under the "Overcoming the Monster" or "The Quest" but only "Comedy*" requires a romantic thread. Of course some stories mix and match these and every so often you get the hero who goes through "rebirth" and becomes a hero from a villianous origin. None, however, dictate there be a romantic interest.
* In this instance, comedy is not "ha-ha" comedy but the original meaning.
edit: Additional detail.