A. While it may sound like a gross or disturbing behavior, it does also sound like a typical and normal thing that a toddler might do.
Of course you understand that it is an inappropriate and unhealthy thing to do, but your toddler can't yet make that connection. He doesn't have any past knowledge about this experience and can't yet predict the consequences of doing something like this. So he was likely just exploring a new part of his body that he just noticed, just like if he put his finger in his mouth, nose, or ear.
Similar 'gross' behaviors include toddlers who play with their poop or put bugs in their mouth.
As with other things, just because it is normal or expected doesn't mean that you shouldn't help your child learn not to do it anymore. And since it is a health hazard, that makes it even more important for him to stop doing it.
Stopping Gross Toddler BehaviorsThe most important thing to understand in trying to get a child to stop doing something like this is that if you overreact, you may actually reinforce the behavior and he may do it more. So stay calm, wash his hands, and simply tell him 'no, don't do that anymore' or 'it's yucky to put your finger in there.'
Remember to stay calm and then watch closely for any signs that he is going to do it again. If he does, try to distract him away from the behavior, like by giving him a bath toy to hold or playing a game.
This also works for toddler who put their hands in their dirty diapers and smear the poop all over.
In this situation, in addition to staying calm and cleaning up matter-of-factly, without getting overexcited, you can try to limit your toddler's access to his diaper. Having him wear clothing that he can't easily get out of or reversing a one-piece outfit often works to limit access to a dirty diaper.