So? You don’t need to be kissing multiple people to get a disease through kissing. Some people don’t even know they have a disease and can spread it to you, even if you’re dating them and kissing them only. You can get herpes from sharing a lipstick or a drink. You can have herpes without knowing it, I can have it without knowing it. Most people have it.
I’m all here for promoting safe sex, but STDS are not the end of the world and are often curable if not at the very least treatable. Oral STDs can be prevented (or lower the risk of infection) with protection. Teaching abstinence isn’t teaching people how to protect themselves, because teenagers and adults are going to do it anyway.