If Tony and Stephen were a thing through out mcu, but Tony still died in endgame. Than comes along America ( he was of course forced to forget about peter) with her big brown eyes, and same sassy attitude. Stephen is constantly in pain about it. And what if wanda promises to bring him to a universe were Tony's alive and their happy as long as she gets America. She totally expects him to agree because their going through the same thing. But they deal with their trauma differently.
continued: If ironstrange was a thing during the mcu: wanda and Stephen bonding over the trauma, but wanda keeps thinking of what it's, and Stephen has come to accept that it had do be that way. Than wanda dangling Tony in front of him but he has to kill a child. A child that he wants to keep safe. Wanda having more to mock him for and call him a 'hypocrite'.
thing is that yes Stephen "loves Tony in every universe" but ultimately he knows that his love, his one and only, is gone. No alternate version of him could ever replace him. Meanwhile, America is alive and theirs and just as irreplaceable. Besides, he constantly finds pieces of Tony in the people around him - Peter, America, Christine even, … Tony thought him to open himself up to other people, and because of that Stephen is still better off having known and lost him. Thanks to Tony, Stephen would never have been able to sacrifice America even if he'd wanted to.
Stephen may never get over Tony's death, but at least because of him, he knows how to open himself up to people and that he is capable of finding happiness and love in a world without him.