From the abstract (highlighting is mine):
Harmful microorganisms can be transferred to hands from contaminated surfaces people come into contact in daily life. Contaminated hands can transmit disease to one self as well as to others. A study was done to determine the extent to which hand hygiene practices and toilet door knobs contribute to the bacterial load of hands of toilet users in a medical school. Swabs were taken from a randomly selected sample of 60 medical students for bacterial count from both hands before and after toilet use and from door knobs of six toilets. Only 40 (66.7%) claimed they washed hands with soap. Significantly more females (83%) used soap to wash hands compared to males (50%). Bacterial load in the hands of both males and females showed an increase after toilet use. The increase was significant among male students. The dominant hand had a significantly higher bacterial load than the other. The mean bacterial load of male toilet door knobs (12 CFU/cm2) were significantly higher than of female toilet door knobs (2.5 CFU/cm2) (𝑃 < 0.0 5). Staphylococcus aureus was isolated from the hands of 21 students. Toilets and washrooms should be designed so as to eliminate the sources of contamination of the hands.
There are differences between circumcised and uncircumcised men regarding the bacteria found on and inside the urinary tract:
Circumcision was associated with a significant change in the overall microbiota (PerMANOVA p = 0.007) and with a significant decrease in putative anaerobic bacterial families (Wilcoxon Signed-Rank test p = 0.014). Specifically, two families—Clostridiales Family XI (p = 0.006) and Prevotellaceae (p = 0.006)—were uniquely abundant before circumcision. Within these families we identified a number of anaerobic genera previously associated with bacterial vaginosis including: Anaerococcus spp., Finegoldia spp., Peptoniphilus spp., and Prevotella spp.
I'm honestly not certain whether any of those fall under the heading of "harmful bacteria". The paper is focusing more on aspects of a health penile microbiome and its effects on HIV transmission.
The CDC have their own things to say:
Feces (poop) from people or animals is an important source of germs like Salmonella, E. coli O157, and norovirus that cause diarrhea, and it can spread some respiratory infections like adenovirus and hand-foot-mouth disease.
But it looks like they focus on transmission through fecal bacteria.
Cecil Adams, of Straight Dope fame, did do a column on this subject, including discussing cunnilingus and fellatio:
What you may not know is that washing will not make the coliform bacteria go away. They're holed up in the pores of your skin and nothing short of sandblasting — certainly not your morning shower — is going to get them out. Showering merely gets rid of the ones that have strayed onto the surface. The bacteria won't do much harm if they stay put, but when you urinate your fingers come in contact with Mister P. long enough for the coliform bacteria in your pores to hop aboard. Your fingers subsequently touch lots of other infectible items. If you don't wash your hands with soap and water (soap gets rid of the skin oil that the bacteria stick to) … hello, Typhoid Mary.
... the truth is you can catch lots of bugs via oral sex. Many of them are transmitted by, or have their transmission facilitated by, coliform or other fecal bacteria or, for that matter, fecal viruses. There's no point in stopping now though. You can catch most of the same germs from intercourse, kissing, or simply holding hands.
Leaving aside that The Straight Dope is entertainment material, and often somewhat inaccurate, take it with a grain of salt, but it fits with what the other sources say, that it's fecal bacteria, that it's all over your genitals, and casual rinsing isn't going to make it go away. Oral sex isn't really adding that much largely because, well, most of us are pretty smeared in fecal bacteria as it is.
I honestly was surprised to find so little actual evidence that hand-washing helped. I knew that anti-bacterial soap has been shown to be no better than regular soap, and that hot water only helps in terms of reducing the amount of soap film on your hands, but I really did expect handwashing to have a higher impact on health in this case. Still extremely useful for surgery, less useful after the bathroom.