Animals large enough to have sex with humans, are strong enough to injury humans significantly, even to kill them. A lot of his compliance argument about animals works with animals because of this hidden assumption. For instance, even if you cannot physically dominate your adult dog you can surely physically dominate a puppy. But since you cannot physically dominate your adult dog, you realize that you cannot extract their compliance through force alone (at least not without drugs or restraints.)

Conversely, a very good reason why children comply more than they should, and why he even makes the argument about children, is that adults are nearly always larger and stronger than children and, ultimately, can physically dominate them through force alone. This of course is worse if and when the parents or other authority figures have been physically aggressive with the children. But it would be unavoidable, even if no adult had been physically aggressive before, when the first one tried to. The size difference is impossible to miss.

When comparing the levels of resistance, he is at least ignoring the effects of strength differential and how they favor the animal against a human but favor the adult against a child.

Although I think it partly stems from general confusion about the issues, because while ignoring the effect of the strength differential he attributes the "compliance differential" he sees to psychological factors: that the animal ultimately isn't loyal but that the child ultimately is obedient. OTOH, this would also tend to argue for pedophilia. If the child is obedient against their self-interest that implies very complex thoughts and necessarily an advanced intelligence. Ultimately this undermines the argument that children are not intelligent enough to handle sex; at least when the comparison point and the presumed cutoff point for enough intelligence are animals.