True. You could get genital herpes from receiving oral sex from someone infected with the oral herpes virus, even when they do not have an actual visible sore.
You can get oral herpes kissing someone with oral herpes.
You can get oral herpes by oral sex with someone with genital herpes.
I have no knowledge of other specific STDs, or their ability to survive and germate in the mouth.
A little quick research tells me: You may get other STDs as well, but in your throat not your genitals. Gonorrhea and Chlamydia both can infect your throat.