I guess I'd have to go back and read - because I suspect that Houjou is effectively oblivious to what Tohno's gone through, in terms of how all the other girls see/think about him (because she is, like, the one girl we've seen who's not after his brother). She's in her own little world it would seem, so that connection between Tohno and Rei might just not occur to her at all.
So she fully doesn't get why he shut down here, on multiple levels. And he's not going to explain it to her.
I look forward to seeing how this bit gets resolved, though. I have no clue how long this series is looking to run (if there's even an actual 'end point' worked out, to that effect), but this sorta reads like a moment where a big checkpoint for Tohno figuring out whether to put his faith in Houjou.