As far as I know. No health problem that a bitch or stud could get, can be solely blamed on a human having consensual non-abusive sex with the dog.
U.I. or Urinary infections? Some dogs get those without ever breeding. (still, wash your penis before putting it inside her, be a gentleman)
Vaginal lacerations? Well, that shouldn't happen with consensual non-abusive sex. So, why would you fear that?
Rectum/vaginal prolapse? Again, dogs can get this without having sex. Constipation, giving birth to puppies, birth defects, all those can cause a prolapse. A vet should have no reason to suspect human sexual intervention. And again, prolapses shouldn't happen with consensual non-abusive sex unless the dog had a birth defect you didn't know about or a past history of getting prolapses you didn't know about, in wich case, you should not be having sex with her/him in the future.
Pyometra? Similar to U.I. Bitches get those even if they never had sex before. FYI: If your bitch is middle age, it may be worth getting her a hysterectomy where only the uterus is removed, so no pyometra can happen. Pyometra risk increase with older age, wcihotu uteris the risl of pyometra is ZERO and she will still enter in heat, even mate with dogs or humans, but no babies can be made as there is no uterus)
Now obviously, don't take your bitch or stud to the veterinarian just after you had sex with him/her. It won't be good if she/he start dripping a fluid that smell like human semen, feels like human semen, taste like human semen and moves and looks like human semen in the microscope.
And in the very odd case you took all proper precautions during sex and a major accident happened during sex that injured the dog genitals in a way that would require medical attention, then you may have to be very creative to come up with an excuse on how the dog got hurt. I'm mainly thinking about males here, I can't see how you could accidentally hurt a bitch genitals during consensual, safe, non-abusive sex.
With males I can see how major accidents could happen, a male penis could be scratched or bumped against a furniture accidentally causing a laceration or trauma. Like, imagine you are holding your dog penis in your hands and peacefully sucking his tip and suddenly 2 cats start fitting all over the roof of your home, your dog instantly runs away to bark at the cats, you had no time to let go his penis, your nails are short and sanded, yet he still gets scratched by your nails pretty bad, not only that, the dog running with his dick flopping side to side, manages to bump his dick into a low table. Oouh. How could you explain a swollen bruised scratched dog penis to a vet? I don't know... Maybe find a vet that is also a zoo in case something like this happens?
