Myth:
You are an atheist so that you can do whatever you want.
Response:
This misconception stems from the assumption that atheists not only cannot be moral without god, but in fact do not want to be moral. This sort of attitude can be summarized by the oft-quoted verse Psalm 14:1: "The fool says in his heart, 'There is no God.' They are corrupt, they do abominable deeds, there is none that does good."
Although those who will quote this verse for you rarely go so far as to include the second sentence, no atheist should ever fail to keep in mind that it is always there, hanging unspoken but nevertheless assumed, in the background. One example of this is Paul C. Vitz who has described "personal convenience" as an important reason why people become atheists, claiming that he could be motivated to reject serious theism so that he could enjoy more sex! He also adds the motivations of avoiding going to church on Sundays and meeting with church groups, avoiding prayer and scripture reading, and even avoiding helping others!
The first thing to note about this is that the claim is nothing more ad hominem attack (specifically, the Genetic Fallacy) disguised as serious thought. Atheism cannot be refuted by attacking the alleged motivations which atheists might have. Even if the above claim about their motivations were true, that does not mean either that disbelief in gods is unreasonable or that belief in any particular gods is reasonable. Using the above claim is not an example of engaging in serious discussion - quite the contrary, it is a means of evading serious discussion.
Vitz labels the above reasons as being adolescent, and they are - but they are also false, and the premise for his argument for dismissing atheism is thus incorrect. Atheists don't avoid joining groups they believe in just because the meetings are a drag, and many atheists love to read - indeed, some have read the Bible more than some Christians.
Perhaps there have existed atheists who rejected belief in gods in order to avoid morality or getting up early on a Sunday morning, but I've never encountered any and the many atheists I have known have had quite different reasons for their atheism. The idea that atheists refuse to believe in a god so that they can save time by not helping others is as insulting as it is absurd.
Thus, the argument for rejecting atheism because of the claim that atheists disbelieve in God in order to do whatever they want is both invalid and unsound. It is invalid because the reasoning process is faulty - it committs a logical fallacy, so regardless of the truth of the premises, the conclusion does not follow. It is unsound because the primary premise is simply incorrect. Whenever you see this claim being made, notice the failure of the claimant to provide any evidence supporting their statements. They are either repeating what they heard because it sounded good or they are just making it up. Either way, it's an indication that they haven't given serious thought to the matter and don't think much about being intellectually honest.
There is an interesting corollary which goes along with this myth, and that is the fact that many theists will refuse to question, much less refine or even reject, their beliefs because they are concerned that doing so will undermine their morality. In such cases, easing their concerns and fears is an important step in helping them to become more skeptical in their approach to religion and god. Thus, it is arguable that many theists believe not on the basis of rational arguments, but on fear of what faces them if they don't - a situation not unlike what the above claim purports to criticize.
Despite numerous attempts, no theist has ever conclusively demonstrated that belief in their god is required for morality. In fact, there are many good reasons to think that such a claim is simply false. There are any number of ethical systems which require no gods at all, and it is not apparent that the existence of a god makes any moral rules either more valid or necessary.
