The voice of experience?
(BTW, according to a quick web search, Z-Pac is an antibiotic, so it's good against bacteria but not viruses)
1 gram of Azithromycin is good for an initial case on gonorrhea and is effective against chlamydia, two of the most most common STDs. And yes, I have given this dose many times to people who have had regrets after moment of weakness of the flesh (boys and girls). We used to give give 250mg of Rocephin IM and 1 gram of azithromycin orally together for those bugs but studies showed just the azithromycin was just as effective.
Yes there are lots of bugs/viruses that may not be covered by this but for immediate treatment this is not a bad choice. Then if really concerned go get tested for everything else. Treatment would be tailored for your particular nasty; HPV, HIV/AIDS, herpes, scabies, lice, syphilis, trichomoniasis, etc.
Jeez when reading this I feel like never taking my pants off again!!
bob