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There are no data to document the precise risk of STDs in this circumstance. But busy STD clinics see few if any patients with STDs whose only contact was the sort described by your patient. In my 40 year career in the STD business, I have never seen a patient with a documented STD, or heard of one in my clinic, whose only possible exposure was hand-genital contact. Not one. That doesn’t prove it can’t happen, but certainly it is exceedingly rare if it happens at all. And the known biology of STD bacteria and viruses would not suggest a risk. Few if any STDs are transmitted by saliva. Some of them – HIV, for one – are killed by saliva, probably one reason why kissing and oral sex rarely if ever transmit HIV. Or STDs.

To the specific exposure, the only infection that would be a conceivable risk is herpes due to HSV1, the main cause of oral herpes. But transmitting HSV is believed to generally require massage of the virus into tissues, not just exposure to saliva or other infected fluids. That’s why initial genital herpes almost always shows up at the sites of maximum friction during sex:  penis head or shaft, labia and vaginal opening in women, etc. In any case, the amount of virus in saliva must be extremely low, even in people with active herpes outbreaks.

Medically untrained persons tend to assume that exposure to just one bacteria or virus particle is sufficient for transmission. Not true. Almost all infections have thresholds of the number of infectious particles and the tissues to which they must be exposed. The very reason STDs are transmitted by sex is that the causative agents evolved – in parallel with human evolution – to be hard to transmit except by large numbers with especially intense, intimate contact with moist genital tissues.

 As for washing soon after sex, it can’t hurt and might slightly reduce the risk of catching anything. But when the risk is as miniscule as in the exposures described, it probably makes no meaningful difference.

 When patients are anxious about STD transmission by means other than intercourse or oral sex, they can be counseled to ignore both the biology of infection and transmission (e.g., whether herpes or HIV can survive in saliva or on contaminated surfaces) and to disregard whether infection CAN occur. Instead, emphasize that it DOESN'T occur; i.e. transmission in such circumstances is rare if it occurs at all; the biological reasons don’t matter. Someone with the kinds of exposures you have described definitely should not be tested for STDs in the absence of symptoms, and in general need not take precautions against transmission to their regular partners. Worrying about one in a million risks – even if we can’t prove they are zero – usually is an exercise in frustration and futility.

My final comment concerns a “sketchy" female partner. Or male partner, for that matter. When the concern is STDs from the mouth, the frequency of infection – oral herpes in particular – is no higher in persons with a hundred or a thousand lifetime partners than those with only one or no past partners.

I hope that helps. Take care.