Tony got used to not breathing after Afghanistan. He remembered the agony of it, the black spots in his vision as the world went out of focus, cut through with angry red spikes of pain. He never thought he'd have to do it again.
But if it was to keep Steve safe, well. There was little Tony wouldn't do. Even if it involved coughing up flowers.
Steve doesn’t mind that Tony doesn’t return his feelings. He just would've preferred if it didn’t come with the side effect of his coughing up flowers and possibly dying.