Judging from her reactions to Senpai's counterattacks, the way she reacts similarly to Senpai when other couples act lovey-dovey, and the fact that she stated in a previous chapter that she thinks other guys are boring, I think that Nagatoro is on the same boat as Senpai when it comes to virginity status as well as never been kissed. It's not the uncommon for someone to tease others about stuff they themselves are guilty of as well. Nagatoro is just projecting some of her own insecurities onto Senpai.
There were so many times he coulda burned her if he didn't like her. I am surprised that the author let him pull the right strings. He almost got a kiss out of it too
...you may have noticed Nagatoro has a certain bad habit of getting carried away and straying outside her own comfort zone. This tends to bite her in the ass on the rare occasions Senpai gathers his wits enough to start calling her bluff.