Some false beliefs are harmless and can lend themselves to casual debate. Some beliefs are dangerous, and dissuading people of those beliefs takes on an added importance.
Moral sense theories fall into the latter category.
Evolutionary ethics is not the only moral sense theory that is out there. Many religious views of ethics put them in the same category. They hold that God has written a true moral code into their brain. Reflecting on how they feel about a particular action is considered pretty much the same as asking God, "Is this right or wrong?" If it feels good to them, then this is taken as God's permission to go ahead and do it.
Common subjectivism is another moral sense theory. Only, the common subjectivist denies that there are any external moral values to sense. Consequently, one's feelings can never be mistaken - there is no fact of the matter to check them against. This, too, leads to the conclusion that "if it feels good to you, then do it."
Evolutionary ethics says that we evolved a faculty for sensing moral properties that we can consult to determine right from wrong. Though individuals might suffer from the occasional "moral illusion" (similar to optical illusions), this sense organ can still be trusted to be reliable.
What all of these theories have in common is that they tell people to answer moral questions by turning their attention inward - by asking and answering the question, "How do you feel about this?"
These internal theories of morality stand in contrast to external theories that say that, to determine moral facts, you have to look outside of yourself at the real world.
Desire utilitarianism, for example, is an externalist moral theory. Instead of looking inside yourself and asking how you feel about something to determine right from wrong, you need to look outside of yourself and ask whether people generally have reason to promote or discourage such a feeling.
If you are perfectly comfortable with the thought of torturing somebody, abusing a child, lynching a black, locking the members of a particular religion in a church and setting fire to it, herding Jews into death camps, enslaving a race, exterminating the Native Americans, raping, stealing, lying, or engaging in reckless conduct that puts others at risk, this is not morally relevant. What is relevant is whether people generally have reason to promote or discourage that feeling.
The reason that moral sense theories are not just wrong, but dangerous, is because it tells people like those listed in the paragraph above that they can trust their feelings when it comes to measuring the morality of their conduct. In telling them this, it gives them a moral permission to act on those feelings. In the cases listed above, this is not a good thing.
Internalist theories of ethics are fine to the degree that an agent has an aversion to harming others, a desire to tell the truth, and aversion to breaking promises, a desire to repay debts, a fondness for liberty, an aversion to punishing innocent people, and the like. That is to say, internalist theories of ethics are fine for people who are already good.
However, it represents terrible advice when given to somebody who lacks a certain amount of virtue.
We can assume that, among the desires that most people have a desire to do that which is right (or, perhaps more commonly, an aversion to doing that which is wrong). If a person has such an aversion then all we need to do is to point out that X is wrong and he will acquire a motivating reason not to do X. It might not always be a sufficiently strong motivating reason. However, in some cases, it will be.
Now, we tell such a person that to judge whether X is right they need to focus their attention on their own feelings. "To determine the morality of your actions you should look inside yourself, at your moral sense, and determine if you are comfortable doing X. If you are comfortable with it, then it is permissible, and your aversion to doing that which is wrong should not be triggered."
Or, we can tell such a person, "How you feel about performing these actions is not relevant. What is relevant is whether people in the world have reason to encourage or discourage people from having those feelings. If they have reason to promote an aversions to this type of action, then you should consider the action to be wrong, and your aversion to doing wrong actions can be triggered."
Of these two options, the first option is going to get people defrauded, robbed, raped, murdered, enslaved, and otherwise abused. The latter option has the potential to reduce some of those frauds, robberies, rapes, murders, enslavements and abuse.
The moral question is not, "How do you feel about this?" The moral question is "How should you (and everybody else) feel about this?"
Moral sense theories – telling people that they can judge moral qualities by measuring their feelings – are not only wrong; they are dangerous. Externalist theories that tell people to look outside of their own feelings at the reason for action that other people have are a little safer.
Just a final note: I am not arguing that a proposition should be considered false if it is dangerous. I am arguing that false propositions exist on a scale - some false beliefs are more dangerous than others, and we have legitimate reason to be more concerned about dangerous false beliefs than harmless false beliefs.