Why didn't Tsukasa point out they weren't actually alone out there? They had Chitose, and then Nasa's students showing up. It's not like there was actual damn privacy. Kaname really just pushes forward like a damn freight train in these conversations, and Tsukasa struggles to keep up.
It is funny as heck, though.
And it caused her to let slip something particularly embarrassing. Seriously, if he's gonna have those sorts of worries, maybe he shouldn't give Tsukasa possible involuntary ammunition to that effect?
Made for a good punchline, in any case.
But just noting this couple's kinda their own worst enemies.