He was upset with her for deceiving him in TWS, yes, but Natasha deceived people, too. Sharon didn't manipulate him while undercover. For instance, when he asked her out, she could have accepted and gotten an "in" to keep a closer eye on him But she didn't, because that would have been messed up. Her job was to protect him (one theory says from himself, since there were indications that he was suicidal). It wasn't unforgivable to him. In CW, he kisses her after she gives up everything to do the right thing - even commits treason. By that point, she's shown she's a loyal SHIELD agent (i.e., not HYDRA), she helped Steve get to Bucky in time to save his life, and she committed treason to help because she thinks it's the right thing to do. So the more he actually got to know her, the more he came to understand that she's a genuinely good person who has to do some things he doesn't agree with. Like Natasha, Tony, or... people.
I think fandom blew Steve and Peggy's relationship WAY out of proportion prior to Endgame. They were never shown to date - as far as we're shown, he never even asked her out until he thought he was about to die, and one kiss does not a relationship make. On top of that, he first met Sharon (and asked her out) before he knew she was related to Peggy - who by then had moved on several decades before. He gets to know Sharon as a SHIELD agent first, and his romance with Peggy was more of a crush that they both got over at that point. Even the writers and some of the cast - prior to Infinity War/Endgame promo, at least - talk about how the whole point of Peggy and Steve was that they never got the chance to be together and that the kiss wasn't even sexy since they had Tommy Lee Jones watching (they also had the Eastern seaboard at risk and people dying around them, but battle kisses are a personal peeve of mine).
We don't know how long they lived across the hall from each other. Sharon and Bucky are the only two characters were we don't see Steve meet them for the first time, weirdly enough. But the original plan for the script - some of the tie-in materials still have this because they were written based on earlier versions of the script - was that Sharon was actually helping Steve and Sam search for Bucky after TWS. Nothing in the film contradicts that, but they cut out anything concrete about it actually happening. So by the time they kiss, they've known each other at least two years, probably longer (it depends on when they moved into the apartment, and how well they knew each other depends on whether Steve actually called Sharon after Natasha gave him Sharon's number).
Steve kissing Sharon was intentional. The writers wanted to show that Sharon was the first person Steve wanted to kiss. She's the first person he explicitly chose to kiss. It also tied into how Steve has learned over time that he's not the same guy who came out of the ice, maybe he doesn't want the white picket fence anymore, maybe he should seize opportunities before he loses them, etc. Unfortunately, at least one scene setting up the kiss was deleted because the Russos didn't like it because it "slowed things down," so we got Steve and Natasha at the church instead. We also have storyboards from Sharon's other scenes that might never have been filmed in the first place.
Don't know if it matters, but that's why some of the stuff is the way it is. Unfortunately, Sharon wasn't the most popular character, and Steve and Sharon weren't the most popular ship, so they didn't have much fandom filling in the gaps and writing headcanons and drawing fanart the way other ships do.
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