Another complicating issue in this is that some STD's don't show up right away, so you could get tested after having sex, it be negative, but you are infected (and, presumably, are infectious). This is the "incubation period" or "Window period" of a disease.
For most STDs it's a few days to a week, but for others it's a lot longer. Syphilis takes 10 to 90 days, HIV can be anywhere from 2 months to 6 months, Hep B is 60 to 90 days.
So you have to bear that in mind when you are thinking about testing plans and safer sex with your partner.
"Listen, or your tongue will make you deaf." - Native American Proverb