Judging by the date of posting it must be the cat story. I'm glad my cats never tried that. All signs within the story suggest the cat is actually an 'imaginary friend'. e.g. Unhappy child, taking cat to school, size distortions.
Cats clearly have SM tendencies. e.g. when they jump on your bed and dig their claws repeatedly into your bare flesh while happily purring.
BTW I thought the other The First Day had a cool, precise style ideal for short stories, but the detached tone did not make it very erotic.
BF