This was because syphilis and gonorrhea were at one time believed to be the same disease and only spread by sexual intercourse. We now know that not to be true.
Secondary contact can also be a source for a few STDs.
They are infections you can get through having oral, anal or vaginal sex with an infected partner. An individual can become infected with more than one STD at a time. STDs can also be transmitted from inanimate objects such as sex toys with fresh and wet contaminated body fluid on it. Despite the stories you may hear, toilet seats are NOT a source of infection. STDs are infections or diseases transmitted by intimate skin contact , exchange of body fluids , or secondary contact .