A dog's mouth is cleaner than a human's mouth.

Comparing a dog's mouth with a human's mouth is sort of like comparing apples and oranges—really filthy apples and really filthy oranges. Both species' mouths are hot, damp places teeming with roughly equal populations of bacteria. Neither would be described as clean, and any question of comparative cleanliness is irrelevant because so much of that bacteria is species-specific. Dogs literally put their mouth on everything; and they use their tongue as toilet paper for not only themselves, but also any other dogs they encounter.