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.
Well, um, technically... There's probably situations when it's a bad idea, but getting a C-section bloodless enough for it to matter might be hard, and people care about babies a lot less than they say they do. Revealed preferences compared to the actual information available to humanity, that sort of thing.
Sure, but really no benefit to baby and only risk so why do it unless there’s some societal pressure
Why do we kiss, hug, and hold or shake hands? It's not for no reason or because we feel like it. There are definite and extremely good reasons and explainable pressures that have led to all of our common behaviors.
Behaviour often comes from evolutionary pressure. Without fully understanding the selection pressures that led to such behavior, and if they are similar or have changed, dismissing it out of hand isn't considering the whole picture.