If was both accept that morality is a social phenomenon, then the answers to the other two questions seem simple enough. An action is moral if it is considered moral according to the standards of the social context in which it takes place. And people "should" behave morally by definition.
Not necessarily. One respected theory of ethics holds that an action is moral if it makes everyone better off on average. Another respected theory of ethics holds that an action is moral if it comports with the Golden Rule. And yet another respected theory of ethics holds that an action is moral if it's grounded in principle, and if it's possible for every other human in the world to follow that principle, too. None of these theories denies that the morality of the action depends on the society around the agent. Yet all of these theories leave room for the possibility that the society in question consider an action moral --- and be wrong.
Hence, the admission that the morality of an action can depend on society doesn't make everything else the slam dunk you think it does.
If one person believes that homosexuality is a barbaric, socially destructive act that should not be permitted and another person believes that a just society accepts homosexuality, how do you decide who is right in any objective way?
According to Utilitarians, you gradually legalize homosexuality and observe if society does, in fact, get destroyed. Observation tells us that it doesn't, and that the would-be criminalizers of gay sex are therefore objectively wrong.
According to the Golden Rule, heterosexuals ask: "If I was a homosexual, would heterosexuals be harming me by hanging me for what me and my partner are consenting to do?". Homosexuals, in turn, ask: "If I was heterosexual, would homosexuals be harming me by sleeping with one another?" The answer, objective again, is that criminalizing homosexuality is immoral.
Finally, according to Kantians, you ask: "Why are these two men sleeping with each other?" (The answer, let's say, is "because they love each other".) "Is it conceivable, then, that every human sleep with a person they love, regardless of sex?" This question, again, has an objective answer, and it is "yes".
Again, observe that the answer to your question always hinges on how the action (gay sex in this case) relates to the society it occurs in. And yet, the answer to your question is always objective. Objectively false, perhaps, but nevertheless objective.
Max Dancona wrote:
I find it funny that moral absolutists have taken both extremes on this issue (and many others).
What's so funny about that? Why wouldn't people be right about moral absolutism and wrong about specific moral absolutes? That's not funny, that's predictable and boring. Humans do this kind of thing all the time.