One of the elementary rules to teach your children is to never bring their fingers to their mouths when they are with others, be they members of the family in the home or strangers in public.
Fingers should be kept away from the mouth or the nose if one expects to be considered to have a minimum of urbanity. There are certain matters of personal hygiene care that are prosaic or grotesque and must be done in strict privacy. Anything involving your fingers in the mouth is included in this category.
This is why if anyone puts his finger in his mouth in public - whether from a lack of formation or as a manifestation of a disgusting humor, as seems to be the case of English historian David Irving above - he will receive a justified anathema from those who praise decorum and civility.