They are often excellent parts, as is the case with Steve Trevor. That film saw Ben Whishaw and Brendan Gleeson take on supporting roles.
Chris Pine's history as The Movie Boyfriend. In both cases, Pine seemed more than comfortable with letting a lady have the cinematic spotlight, turning in charming, memorable performances in both cases. Pine has other big-budget choices, but he still made this one.
I'm not sure if you've heard, but superhero movies tend to feature straight male protagonists, making the category of "Superhero Film — Male Love Interest" pretty damn small, as of right now Hollywood needs to get better at superhero love interests. The cases of both Bucky non-canonical, but easily inferred for the fan who wants to go there and Steve Trevor canonical highlight just how underdeveloped most female love interests in modern superhero films truly are.
While characters like Bucky and Steve Trevor are great characters with their own arcs, the lady love interests of the MCU and DCEU are often forgettable, redundant, and get little to do — despite the fact that they are usually played by some of the best actresses out there.
Most male scriptwriters, directors, and executives are either unable or unmotivated to imagine a storyline for love interests outside of their role as actual or potential girlfriend. There is the occasional exception, like Ant-Man's Janet Van Dyne, aka the Wasp, who gets her own arc that is just as vital as her role as potential love interest. Love interests in superhero films are often just the character who gets to inspire the hero to be heroic.
They are hardly ever the character who is inspired themselves or who is changed to become something heroic alongside the film's protagonist. Even a great character like Peggy Carter in The First Avenger, one of the best of superhero film's modern love interests, doesn't get much of a character arc to call her own until she gets her own MCU short film and then TV series.
In Wonder Woman, Steve gets to act rather than simply react. He also gets his own, fleshed out character arc. He is a cynical American spy who is fighting to stop the war, but who has more or less given up on any idealistic views of heroism or mankind's capacity to be better. Diana changes him and he gets to be a hero in his own right when he stops the bomb that would have devastated London In a movie about heroism, Steve Trevor gets to be a different kind of hero, deepening the themes of the larger story as well as his own character.
We need more superhero love interests like him. But, you know, in female form, too. Look, I don't know how Pine feels about appearing as The Movie Boyfriend in big-budget fare like Wonder Woman or about his past as a rom-com boyfriend, but the fact that he took these roles and performs them so well implies at least a comfort level at being the supporting character to a female protagonist that many actors do not have. Much like Steve Trevor, Pine isn't afraid to stand aside and let his leading lady take the spotlight It also feels worth challenging.