how likely is STD transmission during brief, mutual oral sex

[McNomish]

4 days ago I had a sexual encounter with another guy (we're both late 40's) during which we exchanged brief (around 30 seconds), oral penetrative sex. The exchange of oral penetrative sex was early in the interaction so precum was probably minimal and neither ejaculated in the other's mouth.   I did sucked his balls for a while and licked his penis some, after which I masturbated him with two of my fingers in his rectum.  I didn't notice any sores or lesions on his genitals or mouth.  I recently tested negative for everything and he indicated that he was likewise disease free, but you never know.  How likely is STD transmission during a brief exchange of oral penetrative sex?  Can one likely get an STD from sucking a guys balls and licking his penis?  How about inserting a finger in his rectum?    

[Life360_Dave]

Hi i would rule out the licking for any transmission of herpes or syphilis since no sores were visible and pretty much gono if he had not white puss discharge. all in all very low risk. Test only if you have a long term partner.

[McNomish]

Thank you