I like to believe that Chiyoko came a little after Mafuyu bit her thigh. Chiyoko probably hoped Mafuyu wouldn't notice the clear stain forming on her panties. I don't know how many times Mafuyu kissed Chiyoko's thighs, but I imagine Chiyoko's crotch must be soaked from how much she quietly came during their "session."
Thank you for the translation.