They're saying "ewww" at the baby being pushed back into the vagina (presumably repeatedly, and probably accompanied by other weirdness), not the multiplication tables.

These paragraphs:

"Um..." said the Xenopsychologist. "Okay, I've got to admit, I can't even begin to imagine why -"

Then there was a uniform "Ewww..." from around the room.

"Oh, dear," said the Xenopsychologist. "Oh, dear, I don't think they understood that part at all."

are not about the multiplication tables. They are about something gross that you the reader can only guess at, but a clue is given later during the conversation with Big Fucking Edward:

We do apologize, I say; but who would ever have thought that a species would evolve to find reproduction a painful experience? For us, childbirth is the greatest pleasure we know; to be prolonged, not hurried."

"Oh," said the Lady Sensory in a tone of sudden enlightenment, "that's why the tentacles were pushing the baby back into -"

Frog dissected!