Oral sex it more or less about masturbating a male dog with the added benefit that in the end you do the masturbation with your mouth. So lets begin with how to masturbate a dog. Castration/neutering: The purpose of castration is the removal of sexual behavior. "Another benefit of castration is...
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Castration/neutering:
The purpose of castration is the removal of sexual behavior.
"Another benefit of castration is decreased inappropriate mounting behavior. Mounting can be both a sexual behavior as well as a display of dominance. Neutered males tend to not mount as a sexual behavior because their testosterone levels are much lower than an intact male dog."
(
https://www.petcoach.co/article/benefits-of-neutering-castrating-male-dogs-puppies/)
Male dogs without balls do not have the hormones required for a sexual drive. This results in diminishing sexual interest and eventually complete loss of sexual interest. If a dog had sexual experience before being castrated, it is rarely possible that the dog may have some sexual interest after castration. But it will likely diminish over time. The dog will have more trouble keeping an erection, will ejaculate less and the penis may be more dry and require lube.
If a castrated dog is not responding to masturbation, there is no point in trying it any longer.