It was immediately obvious that Steve had upset her. That was evident in her tone, in the way she practically spat her words out at him. He should have known better, should have realized this would make her angry, but that seemed to be a blind spot for him lately, a mistake he kept making over and over again. And her words might not have been as cutting as Natasha's, but they made him flinch all the same, because no, he hadn't thought about it from that perspective. When he'd kissed her - that had been a moment's impulse, a liberty he hadn't thought he'd actually take until his mouth had been pressed to hers.
He hadn't considered how it could hurt her, to have to watch him walk away after, to what was likely to be a violent, bloody end.
"No," Steve said, "no, it's not, I just - you could still be safe. Even if you were helping me, if you kept quiet enough about it, that might keep you under the radar for a while longer. But if you're on my arm - you gotta know that paints a target on you, Peggy." Surely she realized that? Because it wasn't only that Steve didn't want to have to stand by and watch if something bad happened, although he couldn't even think about that right now without feeling like it might break him; it was that there was still the possibility of keeping anything bad from happening to her. And so long as that possibility remained, he had a responsibility to make sure she knew it.