so if a male dog actively participates in a sexual act with a female human, the dog is not consenting?
What Mr. Jack said. The ability to go along with an act, even to express desire to perform an act, does not imply the mental capacity to provide consent.
When I was a teenager, being male, I would have leapt at the chance to have sex with an adult female. My desire, the fact that I could even have initiated the act, my ability to day "let's do this," all have absolutely nothing to do with whether or not I would have been able to provide consent.
That's why we have laws about sex with kids, whether it's "consensual" or not - kids are not mentally and emotionally
able to give informed consent - to say nothing of the authority that adults carry with respect to children
For yet another example, if I as a sober male have sex with a drunk-as-a-skunk female at a party, I can be charged with rape even if she initiated the act. She was inebriated to the point of being unable to give informed consent - if she has day-after regrets, I'm screwed.
Animals are the same way. They have no way of giving informed consent, even if the animal in question actually tries to initiate the act.