@knight_evangel
Required reading:
https://knowyourmeme.com/memes/subcultures/ahegao
https://knowyourmeme.com/memes/ahegao-double-peace
It's a hentai thing. Apparently, Japanese women lose control of their facial muscles during intense intercourse.
Saki was hoping to mock him with her victory pose, but unintentionally made an expression that Japanese people would interpret as that of a women who
lost (to the intensity of rough sex.)
She's a succubus; of course the author won't let an opportunity for a dirty joke go to waste.