The thing I always find creepy about Steve/Sharon (much as I like Sharon) is that she *reminds him of Peggy*: it's like one of those stories where the vampire, or the mummy, or whatever endless creature fixates on some innocent woman because he believes she's the reincarnation of his long-lost love.

So.

There's your set-up: Cap is an immortal being, a mummy, or a man under a curse, or, hell a sympathetic version of Strahd Von Zarovich if you think it'll work, who believes that some random woman he's met is the reincarnation of his long-lost, not-appearing-in-this-story, beloved.
He's sure of it, it must be true, they are Destined to be together.
And then Tony shows up.
And he's nothing like the Eternal Beloved.
And Cap falls head over heels.