I wouldn't be so quick to dismiss an evolved behavior. Most likely if the parents are OK with it, and in the absence of major artificial factors, the benefits outweigh the risks. The microbiologists are right, but it's just not the whole picture, it's a small part of the whole equation.

My line for this topic depends on familiarity. If you see the baby once a week or more, kiss the stinker if you want to. Otherwise, don't.

That line exists somewhere for everyone, no one would suggest that a mother shouldn't kiss her child.