Sexually transmitted diseases are transmitted through contact with body fluids: vaginal secretions, semen and blood.
However, even saliva, for example in kissing, it can be the vehicle of contagion of infectious diseases (among the most common infectious mononucleosis), but not of those sexually transmissible. You must, however, be careful with any small cuts that may be in your mouth. If in fact if there is an exchange of blood and saliva, this can be dangerous, especially with regard to hepatitis B.