SPOILERS AHEAD for Avengers: Endgame.
In Captain America: The Winter Soldier, it was revealed that Captain America’s love, Peggy Carter, was married and had two kids. According to Avengers: Endgame writers Christopher Markus and Stephen McFeely, the man she married was the Captain America from the main MCU timeline who went back in time at the end of the film.
Speaking to Patrick Shanley of The Hollywood Reporter, Markus and McFeely said that while they can’t decide Marvel’s future plans, especially with time travel now in play, it was their “intention that he was always the father of those two children” after marrying the love of his life.
Source: IGN.com Avengers: Endgame Writers Confirm This Captain America Theory