Firm non believer! I stand with Kant on this one - ethics are objective.
There are a lot of problems with moral relativism/ethical subjectivism. All philosophical ideas go through a period of evolution - ethical subjectivism is no different in this regard. Simple Subjectivism (moral relativism in its most immature form) makes the presumption that any statement about morality can and must be reduced to an emotive statement about personal preference. For example, if I say that abortion is wrong, what I am truly saying is that I do not like abortion, abortion makes me feel things I do not enjoy, or any variation thereof. The most obvious flaw in this belief system is that it posits that such a statement as "I do not like abortion" can be true or false while also claiming that every single person's own opinion must be considered equally true. This creates serious issues when it comes to moral disagreement.
If any moral statement is actually a statement about attitude, and there is no "moral truth," then there is no such thing as moral disagreement. Unless one person can say to another person, "your opinion is wrong" (and subjectivism on the whole denies this possibility), there is no disagreement. Instead, there are simply statements that are not about facts, but about beliefs. This is in conflict with the basic human understanding of the existence of disagreement! If I say that abortion is wrong and you say it isn't, we disagree. Moral relativism doesn't allow for this. Moral relativism says instead that we are simply discussing our attitudes. No resolution can ever take place this way.
Ethical disagreement occurs: I am pregnant. If I say that abortion is morally reprehensible and my acquaintance says that abortion is morally permissible, one of us has to be wrong. This is supported by the fact that no matter what our opinions, a moral choice has to be made. I will either have an abortion or I will not. Furthermore, my action will either be morally correct or morally incorrect. It cannot be both, and it certainly cannot be neither. Ethical disagreement is important to us because it helps us to reason our way into moral understanding. Through discourse, revelations occur. Could society truly move forward if relativism were the best moral attitude? What about rape, murder, child abuse, and torture? Are these things only wrong to some people? Or are they objectively wrong? I say they are. Morality, according to Kant, is something we as humans are able to reason to. It doesn't mean that we always get it right. We stumble, we make mistakes, we disagree.
If discovering a way that one should live and behave is important, then the adoption of relativism cannot be propitious. The person interested in correct moral reasoning must hold an ethical belief system that will allow such reasoning to take place. Moral relativism doesn't allow for this kind of rationalization because it is against the idea that moral principles exist objectively: everyone can be correct about morality because morality is nothing more than a conglomeration of arbitrary emotions. This conviction leaves no real platform for why one 'ought' to do or believe any one thing over another. Individuals need only ask themselves what their 'attitude' is on a particular matter. The theory seems to nullify the whole concept of moral truths as objective and correct prescriptions and proscriptions of universal truths. Determination of right and wrong seems reduced by ethical subjectivism to simple individual whim. Ethical subjectivism tells us that there is no way that one should live.
Whew! That's all I have for now. *grins*