Though dogs may exhibit behaviors that seem sexual after being fixed, things aren't always what they seem. What appears to be intercourse may be your dog engaging in a behavior that he finds pleasurable, or even asserting his social dominance. Intercourse is still technically possible, but your dog is more likely pursuing some other agenda.
Both male and female dogs may be inclined to mount and hump objects or other dogs after being fixed, but that doesn't necessarily mean they're having intercourse in the traditional sense. Your dog may have learned from his pre-fixing lifestyle that humping feels good -- though he can't impregnate another dog, he may masturbate by mounting her. He still has testosterone coursing through his body, so he's still capable of pleasuring himself post-neuter -- even females may engage in this behavior. Dogs also may mount and hump each other to establish their dominance. Again, this is less common after a dog is fixed, but a behavior that you may witness nonetheless. This is not a sexual behavior, though -- simply a social one.
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