My take on this always comes back to Steve's biggest systemic blind spot: his absolute belief that his own moral judgment overrides everyone else's right to the truth. Steve spends three movies criticizing Nick Fury and S.H.I.E.L.D. for keeping secrets and managing people for "the greater good," yet he turned around and did the exact same thing to Tony.
He unilaterally decided that Tony wasn't mature or emotionally stable enough to handle the truth about Howard and Maria Stark's deaths. By robbing Tony of his right to process his own parents' murder, Steve acted with a paternalistic arrogance that completely betrayed their partnership.
Protecting Bucky's innocence didn't require deceiving Tony, Steve simply chose the comfort of a lie over an uncomfortable, necessary confrontation.