I don't understand the argument for 'yes'. Please explain it to me.
I'll bite. I suppose the problem with saying there is no objective morality is that then all morality is subjective - for instance, one could take the subjective relativist position Foucaulf describes above. Well in that case, how do you say that murder, for instance, is wrong? All you can say is you don't like murder, maybe you don't like it a lot, and you really don't want anyone to do it. But if someone else, for whatever reason, really wants to do it, why is their desire invalid, if it is of the same nature as your desire? Two desires of the same nature conflicting, neither is inherently better than the other.
You can't say murder is wrong. That's why we dress up the act of taking another human life in words like murder, or self defence or war or mercy. The taking of another life cannot simply be wrong in itself because as a society, it happens. It is initiated. So you cannot say that taking another human life is absolutely morally 'wrong' because that's not what an absolute wrong or objective wrong
is. The same goes for theft; the taking of an item that does not belong to you. The 'morality' of who you are thieving from and whether they have any stake on that item in the first place is a complex matter in itself. Rape is wrong (though we can only hope that most people think that way). But if you, he casual observer was given a button to subject a victim we perceive to be innocent to either rape or death, we would more than likely choose rape. The alternative is, in it's comparison, less wrong.
And that's essentially why you cannot take morals and line them up against each other. They interact and subvert each other and this is situational. The opposite of the state of murder is 'not murder' which the super majority of us currently reside in. It is a happy state. It is a state that as social animals ensures our survival. Because we don't want to die. We don't want to have to mistrust every person that we see because we could not function emotionally or even sexually. And what is true of us is true of chimps and of lions and so on. There are some species that rely on death and the elimination of everything but the self in order to propagate. That is not a human concern. But if we are jolted out of the state of 'not murder' and face the loss of life or the risk of personal harm or community harm as a real or even perceived threat, then we will make judgements facing that alternative. We may 'permit' murder as a right action, or as a correction to the situation.
Strictly speaking a sociopath or psychopath cannot be considered to be erring against his own nature. However human society has always done it's best for its own cohesiveness to 'remove' sociopathic elements from it, if those elements do not remove themselves first.