It’s just unscientific thinking. People think virus and bacteria are the only thing you have to worry about, but lots of the time it’s the bacteria producing toxins as part of their metabolism that’s dangerous to us. In other words, their shit is poison.
One of the reasons we don’t want some groups of bacteria growing on our foodstuff is because they turn stuff literally toxic to us, completely unrelated to immune responses. Same way some molds can be toxic while others are not. It’s not because the fungus starts growing inside your body and has an epic free for all with your immune system. Its byproducts are just toxic. Like some berries or some plants are toxic.
Toilet licking is especially stupid because different part of our body deal differently with the same bacteria. For example bacteria that are beneficial in your colon are most likely very much detrimental anywhere else. Training your immune system against colon bacteria is beyond stupid. Wouldn’t be surprised if that could lead to all kinds of issues.