"I didn't say you had offered," Steve protested, his blush deepening. He'd never liked how easily his fair skin flushed; it had always given away how he was feeling, whether it was upset or embarrassed or angry. This particular moment was maybe a combination of all three, although embarrassment was starting to win out as he stuttered through a response. "Just that if you did, I wouldn't - it wouldn't be - " Steve abruptly gave up, clamping his mouth shut so as to stem the useless babble he was spouting at Natasha. He was okay with talking to women when the conversation was neutral, but as soon as it veered towards flirting or innuendo, even if it was just the merest hinting, Steve could feel his control of the situation start to slip from his grasp.
For the time being, Steve chose not to argue her point about an actual offer from Natasha changing his mind; he'd no doubt bumble his way through an answer, one which wouldn't help his case in the slightest, and then Natasha would laugh at him some more. He had no interest in providing her with additional ammunition, and so in an effort to get himself back on an even keel, he seized on the flicker of irritation that sprung up at her claim that she'd simply been making small talk.
"I think you and I both know that's not true," Steve said firmly, his gaze stern, demanding honesty from her. Once Steve had caught onto it, the implication had been clear as day. And while he absolutely believed that her intent hadn't been to actually offer him anything, she'd certainly meant to needle him, to leave him feeling upset and unsettled, which in Steve's book was hardly something he would consider simple, friendly conversation.