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 knows that Tony has feelings for him and, since he doesn’t return those feelings, has been mildly dreading the day that Tony decides to do something about it. When that day finally arrives, Steve discovers that Tony’s wooing strategy isn’t anything at all like he’d expected.
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.