Yep I had a random friend of my SIL put his finger in DS mouth when he was tiny and I pushed his hand away and said to DS (actually aimed at the adult) "Yuck DS we don't suck ppl's fingers". Not only is it yuck but you wouldn't go and stick your finger into some random adults mouth so why do it to a baby? Family member is a different story though and I wouldn't have an issue if they had washed their hands.