There is a scene that popped into my head and i can't find the story :(
I think : Steve and Tony had either just got together/were about to get together when Bucky was found.
Tony is a bit insecure, so he decides to do the honorable thing, and backs away from Steve to give him to Bucky. Bucky ends up in the workshop with Tony trying to work out why hes abandoning Tony.
I think Tony blurts out the truth, and Steve was behind Bucky and heard it?
Pretty sure it was on AO3.