Atheists don’t claim that there are any gods and so lack belief by default. Naturally, they avoid the requirement to disprove the existence of God or gods because they don’t claim there are any. On the other hand, if the atheist makes the claim that God and gods do not exist, then they would be expected to justify their beliefs.
Likewise, theists who claim that God exists, are expected by skeptics to justify their beliefs — at least if they want to convince others. There really is no way for a theist or deist to avoid making an implicit claim. Only the soft atheist can claim the default position (of having no position on the existence of God).
The notion of a “default stance”, with respect to a God, gods, or any entities is conceptually the same. Just because someone can claim something exists, doesn’t mean that you necessarily agree or disagree with their claim. It is possible that you don’t know or don’t care about the truth: in either case the fact is that you don’t believe either way. This is analogous to the definition of soft atheism, which is “a”-theism (literally without a belief in a personal God).
Likewise, atheism, being the default rational position with respect to deities, is analogous to amoralism, which is the default rational position with respect to metaphysical moral facts.*
Moralist (noun): those who believe in moral facts and may dabble in moralising.
Moralising (verb): to proclaim moral facts, e.g. “killing babies is wrong”, especially with emotional fervour.
Morality (noun): the set of all moral facts.
As a definition, a moral fact would be a true moral statement. Now, if there cannot be any true moral statements, it is because there are no moral properties in the universe. Put another way: without any moral strand, moral statements cannot refer to anything real in reality. Moral statements become meaningless in a universe without moral qualities. And by definition, if there are no moral facts, then morality does not exist.
For a moral statement to be true, it would need to correspond to a (moral) aspect of reality. It is this (moral) aspect of reality, which would ground the truth of certain moral statements. To be clear, moral components of reality would be moral entities. Now, moralists claim that moral entities exist. It is this claim that amoralists do no accept. By default, amoralists do not believe the claims of moralists.
I guess we could say that hard amoralists claim morality doesn’t exist, and soft amoralists simply refrain from having a belief in morality.
If you believe moral statements like, “killing is bad,” can be true, then you are a moralist. Unless you say this as a completely nonsensical speech act, the root of your claim expresses your belief that the act of killing has a relationship with some moral quality of reality: the act of killing is actually connected to reality in some way other than the physical and conscious act. The important aspect of this moral claim is that there is an imperative to not do it.
The moralist believes moral facts are imperative: i.e. that obligations are upon all humans* to be moral. The moral imperative goes beyond specific moral statements and is broader: not only should we do moral things and avoid immoral things, but we must be moral: morality itself is good.
*Moral persons would be more accurate, as there is debate about what constitutes a morally accountable person: e.g. vegans think most animals are; or another example of some humans (like babies) are not considered morally guilty if they accidentally kill someone.
If we want to claim that “murder is wrong,” without referring to human preferences or situational or contextual parameters, then there must be moral facts. In which case, our feelings about the matter are actually irrelevant. So this is the very question we must ask ourselves: Do moral facts exist?
In other terms, is there at least one moral statement (like “murder is wrong”) which is actually true (or false)?
This of course raises the question of how humans might empirically discover such moral entities, learn moral facts, or rationally deduce moral imperatives.
Unfortunately for moralism, its strongest empirical evidence is intuitionism, after all “it’s obvious that some things are wrong!” But this isn’t an argument, it’s just an appeal to others who already share similar sentiments. Intuition does not establish the existence of a moral rule independent of human thinking: it appeals to examples of human thinking to establish its non-mental origin! (How could mental activity infer non-mental entities? By definition, mental and non-mental domains don’t overlap).
As a moral skeptic, I challenge claims that mind-independent entities that make certain acts obligatory exist. I suspect this entity called morality is just a comfortable narrative that people tell themselves. After all, what reason is there to adopt such a belief?
The amoralist holds the default position, just as the atheist does. This doesn’t mean morality and gods do not exist. It means that I don’t believe in them.
*Not to be confused with empirical morality, of which we can study with science, e.g. people's stated beliefs, common attitudes, etc.