On the other hand, the only alternative is an "absurd" subjectivist account:
One option is that the atheist will defend some sort of strict subjectivism – the type that says, "I don't like rape; therefore, rape is wrong." At which point the theist will simply respond that, "This means you can make rape perfectly legitimate simply by acquiring a fondness for raping others. Slavery, genocide, the torturing of a young child are all legitimate if the agent doesn’t feel bad about enslaving others, slaughtering people, or torturing young children."
Alonzo correctly notes that attributing ethical beliefs to evolution just pushes the problem around.
Either altruism is good because it has an inherent, intrinsic quality of goodness built into it (or it produces something that has intrinsic goodness like evolutionary fitness), or it is good because we have evolved a disposition to be altruistic.On the one hand, where does this mysterious intrinsic quality of goodness come from? On the other, had we evolved a disposition to rape, then rape would be good.
The theist is, however, on the horns of the exact same dilemma, as Socrates argued in Euthyphro. The theist just moves the problem around without solving it by attributing ethics to God just as the evolutionary biologist moves the (philosophical) problem around. The dilemma is fundamental; it does not differentiate between theism and atheism.
But the dilemma is not really a dilemma: The subjectivist horn is typically presented incorrectly. Alonzo makes the key fallacy explicit: "I don't like rape; therefore, rape is wrong." This is a non sequitur fallacy: the conclusion cannot follow from the premise. We cannot deductively conclude an objective property from a subjective property; we are adding something in the conclusion (objectivity) that is not present in the premise (subjectivity).
We could use the scientific method: We can hypothesize that rape really is wrong and therefore we don't like rape. But this simplistic scientific hypothesis is quickly falsified by the fact that some people (i.e. rapists) actually do like rape. We must come up with some hypothesis that explains the fact that most people don't like rape but some people do, and how to tell the difference between such people.
One obvious feature of objective physical truth is that the truth is true and knowable regardless of how people feel about it. Nobody likes childhood leukemia, but it is manifestly real. Everyone — at least everyone who has tripped and taken a bad fall — would prefer that gravity be variable and subject to our will, but it keeps pulling us towards the Earth regardless of our convenience. And yet we see nothing like that in our ethical beliefs: There seem to be no ethical beliefs — at least no secular ethical beliefs — that are strongly held contrary to the desires at some level of the majority of people.
Oddly enough, it is the bizarre, seemingly arbitrary religious ethical beliefs that give the strongest support for objective religious ethics. We must, for example, execute our disobedient children by stoning regardless of how disgusting and barbaric we believe such an action to be. But Socrates' argument still stands: this view doesn't make ethics objective, it just makes ethics a subjective property of God. The theist hasn't eliminated subjectivity, he's just concentrated the subjectivity in a single agent.
The way out of the dilemma is to say, "I don't like rape." Full stop. There's no "therefore". The theist's objection falls flat: "If I liked rape, I would like rape, duh. But it's a fact that I don't like rape." If we all liked eating pie, we would eat pie; most people do in fact like eating pie, and most people do in fact actually eat pie. No problem.
We can also observe that major changes in our legal and ethical beliefs are preceded by a change in subjective attitudes. We did not eliminate slavery until after people started to subjectively find slavery to be abhorrent. (And after the economic benefits of slavery became irrelevant.) We are now in a struggle to extend basic civil rights to gay people, and this struggle is driven by the fact that people's subjective attitudes towards gay people have changed: Those people fighting for gay civil rights are just those who no longer find homosexuality abhorrent.
Eliminating the "therefore" means that our socially constructed ethics, morality and law do not fundamentally reflect an understanding of how society "should" be; they are, rather, an expression of what people actually want (and an understanding of how we can best satisfy our desires constrained by physical reality and imperfect knowledge).
Furthermore, there's a feedback system: Our prior social constructions influence (in complex ways) what we presently want, and what we presently want influences (in complex ways) our future social constructions. these social constructions are therefore dialectical: Everything is "bouncing around", not just unfolding in a linear, easily predictable way. Rather we see conflicts (contradictions in Marxian terminology) between social constructions, actual desires and changes in our understanding of objective physical reality. These conflicts, and their resolution, drive changes in the social constructions of ethics, morality and law.
We no longer need ask that evolutionary psychology* give us a philosophical justification for our ethics, we need only ask this science to give us a causal explanation of why we actually have some of the desires we do in fact have. Likewise, we're equally freed from drawing philosophical conclusions from our causal history: If we evolved to have certain desires, those desires will have in the past influenced our present social constructions. But if our desires have changed, if they are in conflict with our present social constructions or our understanding of physical reality, the causal history is irrelevant to the dialectic. In just the same sense because we evolved to die relatively young does not now mean that our present desire for long life is somehow immoral.
The atheist's desire for an objective morality is nothing more than a holdover from our theistic days, the desire to disclaim personal responsibility for our actions.
The "horns" of the dilemma resolve into simple truth and falsity, between an scientific understanding of psychology and sociology on the one hand and mysticism and bullshit — theistic or atheistic — on the other. We humans are beings with desires. Rationality is shared; nothing else but our idiosyncratic desires define us as individuals. And we are all stuck in a room with each other: We have no choice but to engage in negotiation and propaganda to try to live together as best we can.