NOTE: Spoilers for that movie everyone on the planet has seen by now!
As we all know, in the final moments of Avengers: Endgame, Captain America travels back in time to return the Infinity Stones that he and his pals pilfered from history. But he never returns -- or rather, he doesn't return via time travel. Instead he pops back to the 1940s to live the rest of his life with his true love, Peggy Carter. After taking the long way back to the present, he's a time-ravaged senior citizen, presumably condemned to be a geriatric for another 300 Kafkaesque years thanks to the super soldier serum. But does that contradict what we've already seen in the MCU? In Captain America: The Winter Soldier, Cap learns that after he went full Encino Man in the North Atlantic, Peggy moved on, got married, and even had two children.
So did Cap's actions change the timeline and preempt that marriage? Do those kids even exist anymore? One popular fan explanation holds that the Steve was Peggy's husband all along. After all, we never see him in any photographs. And Endgame screenwriters Christopher Markus and Stephen McFeely recently confirmed that theory, stating: "It was always our intention that he was the father of those two children. But again, there are time travel loopholes for that." Which on one hand is kind of touching ... but on the other, it also creates a whole bunch of weirdness. (And we're not even talking about how Cap frenched his own niece.)
In order for Cap's presence in the present as an old man to make sense, he must not have done anything to alter the MCU's history, avoiding creating alternate timelines the way the Avengers' sloppy time heist did. Now, so far this tracks, since his future with Peggy was already her past in The Winter Soldier. But this also means that in order to preserve the timeline, Steve had to make some big concessions. To not disrupt the past, he'd have to keep a low profile, and wouldn't be able to interfere in world events. That means allowing over 70 years' worth of major tragedies to occur. Which you have to imagine would be an agonizing experience for an incorrigible do-gooder like Steve Rogers. He would also have to let his wife continue punching the clock at an organization he knows is secretly run by friggin' Nazis.
At least Markus and McFeely noted that this also means that "there are two children who have somewhat super soldier DNA." So we can all look forward to movies, TV shows, and for all we know condom commercials starring Captain America's vaguely superpowered offspring.
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