The freshmen see that there are many people who disagree about one detail or another of their childhood moral codes, yet appear to be decent people, or at least have not been struck down by thunderbolts. So, being decent folk, they begin to emphasize tolerance for the variety of moralities they see, and relativism seems to be the best format for this tolerance. But not everything is as innocuous as sexual preferences. No logical problems here. As you can see, though, relativism does take a risk.
Relativism can become moral nihilism in the same way that divine command can become absolutism.
Nevertheless, relativism is the moral theory followed by the majority of people in the hard sciences, including the more experimental, physiological side of psychology. The third main category of moral theory is moral realism. Moral realism says that good and bad, right and wrong, exist in some fashion in this world, and independently of things like social customs, beliefs, or opinions. Moral realism is the middle ground between the theological theories and moral relativism, and is the most common approach of philosophers. But, as is usually the case with the middle ground, that is not an easy position to take.