Kissing, licking, sucking, French (kissing), blow job, oral sex – lots of different names for the same thing: sex using the mouth, sex using the tongue. This involves saliva. You ask yourself if there's any risk in contact with saliva?
As always, if everyone involved is healthy – no problem. But if you're not sure, saliva can certainly contain agents causing sexually transmitted diseases. Mostly the concentration of these agents in saliva is not very high. So no panic. And even if there is something present in the saliva, it has to find its way into your body for you to be infected. If your skin is completely undamaged, these pathogens can't do you harm. But sometimes even minute injuries are enough. The skin on the penis, especially on the end, the glans, is particularly sensitive. And soon damaged, without you even noticing it.
So what might there be in saliva and how can you protect yourself?
The pathogen causing hepatitis B, HBV, can actually be present in the saliva of infected people. If you have sex often and with a lot of women (or men) you should get yourself inoculated against hepatitis B.
Herpes viruses aren't present in saliva but are often on lips. So you can become infected just from kissing and from oral sex.
In addition, pathogens causing clap (gonococci) can be present in the throat, because they like a warm, moist environment. And you can also pick up an infection from a syphilis-ulcer in the mouth.
So if you let someone lick, tongue, bite or suck you, your best protection is to wear a condom.