Cap's character (spoilers)
Cap's decision to reunite with Peggy and change the Timeline is so out of character because
He leaves the Avengers, his family. Remember what t he said at the end of Age of Ultron "marriage The guy who wanted that went into the ice. Another man came out"
He could have romance with Sharon Carter that would not interfere with his Avenging
Two, he would respect Peggy Carter's marriage and not presume that he would be a better husband for her, even if it it is in another timeline and he doesn't break up her marriage in that timeline
Cap was friendly with Peggy when she was older. Their relationship changed. He would not disrespect that changed relationship (and the timeline-see below) just to seek his own satisfaction. He would have pursued a relationship with Sharon (which Peggy would probably approve of)
Third, Cap would not jep the timeline in order to pursue his own gratification. It alsmost is like he cannot accept life the way it is, with its trials and sorrows , but is going to go back in time to escape them. This is WAY out of character for this World War 2 hero. It is like he is trying to escape into his own reality the way people escape into non-reality via, drugs, video games, etc.