A girl I was dating many years ago, when we were first together, I used to put my fingers into her mouth lots, just in everyday situations.
One day, when walking in the park together, I did this and she bit down on it properly hard and then smiled at me as if that was a cute thing to do.
My finger was bruised for about two weeks after that.
Suffice to say I learnt not to put my finger inside a girls mouth again.