Objective morality means nihilist morality. And that's fine. Partly because everything is fine, but mainly because logic is irrevocable, so even nihilism has rules.
Basic nihilism: nothing is necessary, everything is permitted.
However, this means whim implies ought. If you want X, you ought to commit act Y which leads to X.
Since all whims are valid, some whims are not valid: the ones that are explicitly about invalidating someone else's whims. "I don't want a cookie, I just want you to not have a cookie." This whim invalidates the justifying principle, and thus logically invalidates itself. The principle says I ought to make you ought not do what you ought to do. Bzzzt. If it is okay for me to suppress your whim, it must likewise be okay for you to suppress my whim, in particular the whim about suppressing your whims, and thus my whim is self-delegitimizing. If it's false it's false and if it's true it's false. In all cases, false. Strictly speaking such things are not whims at all.
When I say my course is Right and Moral and Good, I contradict the
[everything is permitted] clause, and thus invalidate my own
validation. "It is Wrong for you have cookies." Put colloquially, anyone saying you ought not to do
something because it's selfish ought to be punched in the face until they shut up. This also applies to your own whims invalidating your own whims.
Because cooperation is always rational, cooperation is always possible, and defection can be very clearly defined: the act of trying to make cooperation be seen as irrational. In practice it's always possible to bake enough cookies to satisfy both of us. Defection is attempting to nevertheless dissatisfy one of us, which encourages everyone who is being actively dissatisfied to declare war and destroy the defector.
Lies are also self-condemning in this way. "I'm not going to take a cookie," as a strategy for getting a cookie. "Getting this cookie requires saying I shouldn't get this cookie." Bzzzt. "It is right for you to oppose this whim." Okay, will do. When you support the opposite of what you're doing, you invalidate your own course of action. Or: Kant was onto something but didn't quite make it.
In both cases remove the whim causing the problem; invalidate it. It doesn't count as a real whim. Remember, ought implies can. Whims that are unsatisfiable also don't count. "I want to eat every cookie in the world."
Nevertheless, objective morality still isn't real objectivity. The point of objectivity is to stop arguing about it. Take out the ruler, get a measurement. Here, it is impossible. There's a bunch of finesse involved. Technically anyone can argue that a yard stick isn't a yard long, but in practice that's stupid. For morality, it should be stupid but almost never is.
Because there are limited resources, it's easy to cast someone's whim as invalidating your whim, even if it inherently does no such thing. "I want six of the ten cookies, therefore your desire for five of the cookies invalidates my desire." It's not like starting with eleven cookies was logically impossible.
Similarly, even if we had nice objective rules about that, it would be easy to do the opposite, and cast a whim-invalidating whim as merely a resource-competition whim. "I'm not trying to starve you of cookies, I just want all ten, that's all, see..." It's not like starting with fifteen cookies was impossible, but we already didn't do that, creating a loophole. More generally, it would have been possible for humans to evolve such that they're fully satisfied as a set without requiring more resources than exist, but that already didn't happen.
As a practical matter it is necessary to decide beforehand on bright lines which demarcate trespass. Property lines, if you will. Boundaries, even. "These are my cookies, so they go to who I say they go to. If you don't like it, bake your own." However, it must be possible to argue with the boundaries. It is possible, indeed easy, to create a positive right - property ownership over a whim to directly suppress someone else's whim. "I don't have my own oven so I get to bake my cookies in yours if you're not using it." You just want to be alone today but it's "trespass" not to let me invade your house and dirty your oven.
But, because property must be arguable, it can go in the reverse direction. Toward more defection, instead of more cooperation. Maybe at first I had to pay you a cookie tax to use the oven, but now I get to use your oven even if you were using it first and sit on your couch watching your TV. Easy to argue that a rent is actually a subscription.
This is why it's necessary to have Exit. To be able to unilaterally (but forthrightly) declare someone a non-cooperator, and thus absolve yourself of the requirement to cooperate with them. This is still fuzzy, but Exit, at least, must be perverted to do bad, rather than requiring perversion to do good. The slippery slope points in a responsible, cooperative direction.