Markus and McFeely accept that different people will have different viewpoints on this topic. But, in their minds, Steve was Peggy's husband all along.
"It was always our intention that he was the father of those two children. But again, there are time travel loopholes for that," said McFeely.
Added Markus: "It does introduce the idea that there are two children who have somewhat super soldier DNA."
https://www.hollywoodreporter.com/heat-vision/avengers-endgame-writers-confirm-captain-america-fan-theory-1209241