What if Bucky never fell off the side of that train? What if he'd joined Steve on Red Skull's plane, and they'd figured out a way to get out of it alive and not frozen? What if they'd both made it back safely from the war? By now it's almost the 50s. Steve marries Peggy, and Bucky finally settles down with Private Lorraine. Since Howard never suffered the loss of Captain America and became a womanizing drunk, and the Winter Soldier project never took off, him and Maria live good long lives, and have several more children after Tony. Obadiah gets caught stealing from the company and fired, when Tony is still just a boy.
Fast forward twenty-five years, to the early 70s. Since his dad was never messed up by Steve going missing, Tony has managed to grow up into a well-adjusted individual. Tony has taken over Stark Industries, and already made the company's new focus clean energy rather than weapons. Of course, he also has a few side-projects... it's hard not to, growing up with Captain America for an honorary uncle and godfather. In a few more years, he'll marry Pepper, but she's not quite ready to settle down yet. She has a business degree to finish first, and her ambition just makes Tony love her all the more.
When Steve gets a bee in his bonnet, everyone agrees to march in the first ever Gay Pride Parade in San Hose, 1974. In a highly controversial move, he marches in his Captain America suit. At the end of it he makes a speech, saying that he has always stood for Justice and Equality; for women, for people of colour, and anyone else who might be oppressed, and he didn't think that people should be treated badly just for loving someone of their own gender.
Bucky and Lorraine's oldest daughter, Becca, has married the love of her life. Steve and Peggy's only child, Jane, has married Tony's younger brother. They'll both have more than one child, but the important thing is that in the early 80s, they'll get pregnant at almost the exact same time, and both have one last child. They'll both have boys. Becca will name her son James, but everyone will call him Bucky. Jane will name her son Steven, but everyone will call him Steve. As they grow older, everyone will watch with affection and amusement. They are the spitting image of their namesakes, and have clearly inherited their personalities as well. Steve is a little short and skinny, but fortunately not as underfed or sickly as the original had been, and he still has a Bucky to take care of him, and watch his back in a brawl.
Fast forward one last time, to the new millennium. Bucky Jr and Steve Jr watch in awe as Massachusetts becomes the first US state to legalize same-sex marriage, followed a few years later by Canada. California and Connecticut are next in 2008, only for the law to be overturned in California later that same year. Steve clenches his jaw in anger, and Bucky side-eyes him, wondering just how he's going to keep Steve away from the protesters during that year's Pride.
The next year Iowa, Vermont, and New Hampshire join the rainbow states. When New York follows in 2011, Bucky slips away while Steve is whooping at the TV. When he comes back into the room he's blushing, something Steve has never seen him do before, and holding out a box with two rings in it. Captain America, their last living grandparent who is just now starting to get grey hairs, cries at their wedding.
AN: Just in case it isn't obvious, I totally ship Stucky. I ship it pre-serum, post-serum, pre-war, during-war (the Commandos all knew and shipped it too) and post-winter soldier. I ship it any time, any place, in any case. ^_^