While I agree that if a dog isn't interested, it isn't interested and you shouldn't force it, saying that putting peanut butter on your dick is borderline rape is just absurd. If you put peanut butter on your hand for a dog to lick off, would that be rape? You're enticing the dog to lick a part of your body using food, so from the dog's perspective, it's exactly the same.

Treating activities differently just because they're sexual for the human involved is exactly what hysterical non-zoos do when they condemn us for giving a dog a handjob but are totally fine with semen collection for artificial insemination. It's the perspective of the dog that should determine the morality of the act, not the mental state of the person performing said act. If the dog is happy to lick peanut butter off a spoon, or your hand, or your dick or whatever, then I can't see a problem (except possibly that feeding dogs a lot of peanut butter might be bad for them, I dunno, I'm not a vet).

When a dog licks a guy's cock, it does so because it's thinking "this thing is tasty and so I will lick it," not "I will lick this guy's cock because I have decided that licking cocks is what I want to do." It's not a moral or ethical decision on the part of the dog. Boundaries don't really come into it. If it's not licking, it's because it's not interested in the taste. Rape would be forcing the dog to do something despite it not wanting to and trying to resist; not engineering a situation whereby it does something of its own free will that you happen to enjoy. You might as well call it rape to teach a dog to sit or roll over using food treats as a prompt or reward Both involve getting the dog to do what you want it to do of its own free will by using food as an incentive, but no-one would object to the second one, I'm sure.