It's a lesson taught by some religious (e.g. Sunday School) teachers. It goes something like asking a group of kids "well you don't want to eat a cookie that has just a little bit of poop, would you?" Relating it to some "worldly" practice, from my hazy ADHD memory I seem to remember it being swearing in my lessons, but there's many variations I heard from other people.

So the idea is "just 1" worldly thing is 1 too many and you must stay 100% pure for the big G.

In this case, she's saying the metaphorical poop in our society "cookie" is trans people existing.

It's utter christofascist puritanical horseshit.