The MCU is no stranger to fans who love stan and ship their characters. Some of the ships, like Iron Man/Tony Stark and Pepper Potts pay off beautifully but often, they exist in nothing except for fanfics. There are a few ships that have made a bit more of an impact than others, and it seems like with every MCU works there's a new couple that the Internet is going crazy for.
With the launch of the MCU television series on Disney Plus, there is now more opportunity for long-form storytelling. This also means more extensive character development within the same stories, as well as better relationship development. While their first show, WandaVision, focused on an already established couple there wasn't all that much opportunity to ship. However, then came along The Falcon and the Winter Soldier which of course focused on Sam Wilson/Falcon and Bucky Barnes/Winter Soldier.
The series really had a pretty heavy emphasis on Sam and his character development, of course culminating in him stepping into the role of Captain America. Throughout the series, there was a bit of a conflict between Sam and Bucky because Sam initially turned down the Captain America shield which was against Steve Rogers's wishes. Because these two men were the focus of the show and had a lot of screen time together, as well as a bit of a banter, a lot of fans naturally began shipping them as a couple. Though the show has been over for some time now, it's still talked about on social media and gets quite a bit of attention. However, this is a ship that just isn't going to happen.
The main and most important reason that no one should expect a Bucky and Sam romance, is because everyone involved has said that it will not happen. Though Sebastian Stan, the actor behind Bucky, hasn't discouraged the shipping, he also has not said that it's happening. And most recently, Anthony Mackie (Sam) got into a bit of hot water for his vocal discouraging of the Sam/Bucky ship.
Speaking in an interview with Variety, Mackie pushed back against the idea that Sam and Bucky would ever be in a relationship. "Bucky and Sam have a relationship where they learn how to accept, appreciate and love each other," Mackie said, "You’d call it a bromance, but it’s literally just two guys who have each other’s backs."
As much as the Sam and Bucky shippers might want to acknowledge that, he has a point. It may seem a bit deep but this is kind of a societal issue. Because of toxic masculinity that penetrates so much media, it's hard for people to see two men care for each other and not think they're in love. Throughout the show, Sam and Bucky really do have a strong friendship that could be interpreted as loving. But is there ever any actual, sexual, or romantic tension? Not really.
A scene early on in the show where they were doing group therapy that might have slightly resembled couples therapy was the only real moment that exposed any of that. Many would say that's also the scene that sort of sparked all of the talk regarding Bucky and Sam as a couple. Upon watching, it really isn't romantic as much as it is funny. And otherwise, yes they get along and have banter and care for each other, but it's not portrayed in a romantic way at all. If anything there is a moment towards the end of the series where they allude to Bucky flirting with Sam's sister.
Looking outside of toxic masculinity, there is also an issue of representation (or lack thereof) that exists in the MCU and media as a whole. There is a noticeable lack of LGBTQ+ representation, and that leads fans to creating their own couples that fall under the queer umbrella. Even within that representation, there's a noticeable lack of representation for gay couples who fall into "masculine" personality and physiques like Bucky and Sam do. And even further than that, there's a specific lack of representation for gay men of color, Black men especially, who are masculine. Because this isn't shown, young people who are LGBTQ+ or older fans who have never seen themselves represented are inevitably going to continue to try and find something they can identify with.
The fact that the Internet and social media are so strong and those spaces are really where fandoms are flourishing just emphasizes all of this. And of course, some light shipping or wishful thinking is completely harmless. However, some of these explicit fanfics and coming for the actors or writers asking for a couple to be formed can be seen as inappropriate and in some cases, harassment. The lack of LGBTQ+ representation is especially noticeable within the MCU and while there are hopes for that to change, Bucky and Sam is not where hopeful viewers should be looking.