Cavities are caused by bacteria. The bacteria lives in saliva, kissing and food sharing transfers saliva. So people who say kissing causes cavities aren't just making up stories out of thin air. My problem with the "no kissing" theory is that I don't know that avoiding kissing and food sharing is really a sure fire way to avoid the bacteria entering a child's mouth anyways. Bacteria are sneaky, common, and persistent. It sounds like a massive amount of effort to avoid something that is eventually going to happen anyways. One quick kiss to your toddler from great Aunt Sally at Christmas time and YEARS worth of meticulous sterile conditions would be out the window. When a persons mouth has been colonized with bacteria once, it is nearly impossible to eradicate that organism entirely. The bacteria in our mouths tend to live within a bio film situation, and while you can suppress it you rarely kill it completely. It becomes a parasitic part of our normal flora.
On top of that - eliminating bacterial colonization within our bodies isn't necessarily a good goal. There has been research to show that a lack of bacterial and/or parasitic growth in our bodies is partly responsible for the higher instance of autoimmune diseases and underdeveloped immune systems in developed countries as opposed to those living in developing nations. While cavity causing oral bacteria might not be the poster child for that concept, it is a bad habit for parents to get into to attempt to keep their kid sterile.
So my feelings on the subject are that your best bet for cavity prevention is regular and thorough brushing of teeth, as well as avoiding excessive amounts of sweets, juice, etc., that promote excessive and rapid bacterial growth between brushing.