For a bit of history, dental dams first became popular during the height of the HIV pandemic, when we lacked good data about how the virus gets transmitted. Anyone performing oral sex was urged to use a dental dam. Then, after a lot of years and data collection had gone on, scientists started realizing that oral sex was barely a risk factor for HIV. (In fact, it's now thought that only a tiny handful of HIV cases, if any at all, were caused by viral transmission during oral sex.) So, HIV educators (like me at one time a long time ago) stopped focusing on dental dams in favor of focusing on messages that seemed more critical, like "get tested and treated," and "make sure your partner wears a condom if he penetrates you anally or vaginally."

Then, because dental dams only really became a thing because of HIV risk, lots of people overlooked that they can also be very effective in preventing other STIs.