Well said.
I simplify my argument in this capacity.
While it is much more difficult to determine "consent" from the entity being penetrated or physically stimulated, consent is implicit by the penetrator (unless coerced). This is true regardless of the entity and is applied universally across the board.
So in the argument that animals cannot consent, I bring up a situation that would likely still be viewed as both illicit, lewd, and illegal under the law.
[Situation where consent is inherent]
If a person is minding their own business and an animal mounts them, has the animal given consent? In this situation the person has not provoked or encouraged this behavior. Choosing to do nothing is a valid choice, the same as letting a dog hump your leg. So if the animal mounts and penetrates and the subject being penetrated does not attempt to stop this action, then both parties are inherently consenting, as both are acting on their own accord and are free to stop/leave.
The argument of determining consent of the penetrated or stimulated is much more difficult.. To use dogs as an example:
[Situation where consent is uncertain]
If a dog is able to be penetrated without restraint (meaning it is free to leave at any time) and it has not been trained to accept this behavior as a means to getting a reward, then an argument could be made more easily that "consent" is possible. -- It's simply a lot harder without the communication (verbal or written) skills we use with people or in the clear conveyance of non-verbal skills by people. But if a dog is being restrained, trained to accept this behavior using a reward or punishment system, or is being stimulated orally, then it's much harder to determine consent - even if consent was given previously, (ie in the case of a dog mounting where consent is implied, but then receives oral).
[Summary]
In cases where the animal is subjected to pleasure by a person, the issue of consent is an argumentative and uncertain one.. in contrast, cases where the animal freely acts on it's own behalf (ie licking genitalia or mounting) there is often clear and inherent consent.