Can we please lay off calling people who do not like the whole Sharon Carter/Steve Rogers relationship names? Not all of us are disappointed that Steve and Buck didn't knock the bedposts. I'm not. I even know that Steve and Sharon had a fling in the comics. But here's the thing.
1. Marvel movies and Marvel comic books are different for a variety of reasons. It's Wanda not the Scarlett Witch, right? Black Widow and Hulk, anyone? And wasn't Bucky younger in the comics? Seriously, the "well they were together in the comics" is a bit overdone, we are talking two different formats. Marvel movies are not geared just towards comic readers. (thanks to Grimlock for pointing out the problem with that last sentence)
2. In terms of the movies, the relationship developed almost completely off screen. It's true that in WS, Cap and Sharon flirt. And it is to Sharon's credit that she brushes him off gently, one presumes because of her assignment. But in WS, Cap doesn't know her real name and his last conversation with her isn't a conversation. It's "You betrayed me" neighbor. We go from that last interaction to Sharon betraying the people she works with for Cap with NO REASON given in the movie universe -unless you count a relationship or a really bad and unhealthly case of hero worship.
3. When Steve gets the message that Peggy died, at first I assumed that Sharon sent it. If that had been the case, then he knows about the relationship and it would indicate the two talked and made up. It is made clear at the funeral that this is not the case.
4. Because of 3, the flirting post funeral comes across as disgusting. For me, it was disgusting on Steve's part. I'm sorry, I don't care if she was from outer space, flirting with someone right after burying a former flame is icky to me. Sorry.
5. The really great thing about the movie is that both Cap and Iron Man are right and wrong. Ideally, the viewer should most likely feel like Widow. Outside of characters like Ant Man and Spider-Man, you can feel the conflict in terms of no one really wanting to hurt anyone. In many cases, we also know why people side where they do - Tony out of guilt, Rhodey because of hierachy (and his story arc is heartbreaking), Widow because she believes in some oversight, Sam because he doesn't fully trust the government. But why does Sharon betray people to help Cap, especially after the Bucky tried to kill her? The only motive that is hinted at is the relationship one, but what relationship have you seen? In the movie, we are treated to the deep relationships between almost everyone, but no real development on the Sharon/Cap relationship but the flirting. Seriously, that's why she does it? (And honesty, if you were Sharon, wouldn't you be asking is it me he wants or my connection to Peggy?) (And even Ant Man hints at another reason to support Cap than his his hero worship, he mentions what Pym thinks of Stark).
6. Look, I don't really buy the whole Widow/Hulk relationship either, but if you go back and watch the first Avengers movie, they had more alone screen time to develop a relationship than Cap and Sharon did. Hell, Black Panther and Widow in this movie do.
7. It's not the relationship, it is Sharon only being defined by that relationship and that relationship has not been well shown in the film universe. Hell, even Hawkeye's wife who had less screen time and came out of nowhere, had more development as a character. Take out the kiss, and Sharon's part could have been fulfilled by Coulson.