Update: I fixed some serious formatting issues with this post.
There's an argument that goes, if God didn't exist, then there would be no reason for us not to do action X. But action X is obviously terrible, so God's non-existence would have terrible consequences.
This is a fairly weak argument, since it's an argument from ignorance.* Just because you can't think of a reason not to do action X, doesn't mean there is none. What's worse, the argument undermines itself. On the one hand, X is supposed to be obviously wrong; on the other, we supposedly have no reason to avoid X without God.
*It's also an argument from consequences, but let's focus on one fallacy at a time.
At this point, a proper response requires splitting into three different cases:
1. Action X is obviously wrong whether or not God exists.
Examples: murder, rape
This is disingenuous. If it's so obvious that X is wrong, then this contradicts the statement that there is no reason to believe X is wrong. Obviousness is a reason for belief.
2. Action X is obviously wrong if God exists, but not wrong if God does not.
Examples: same-sex sex, evolutionary science (depending on the religion)
But if X is not wrong, then why would it be bad if people do X? It seems that the terrible consequences of God's non-existence aren't so terrible after all.
3. Action X is obviously wrong if God exists, but not so clearly if God does not.
Examples: euthanasia, late-term abortion (also depending on the religion)
If an issue is truly uncertain, then it is appropriate to be uncertain about it. Why should we afraid of lacking hard answers if no hard answers appear? Isn't it worse to think we know the answer when in fact we don't?
Note that some people will claim that stuff like murder is uncertain without God, but we should take these claims with a grain of salt. If they truly thought it was uncertain, they wouldn't think it was bad for people to feel uncertainly about it.
In conclusion, the basic argument from morality is fallacious at best and disingenuous at worst. Other kinds of arguments from morality are outside the scope of this post.
An explanation of the title: There's an expression in philosophy that goes, "One man's modus ponens is another man's modus tollens", describing a common impasse in arguments. Both parties agree that A implies B, but one person thinks A is obvious, and therefore B is true (by modus ponens), while the other person thinks B is absurd, and therefore A is false (by modus tollens). This describes the situation with the argument from morality, but if you can't see the connection, don't worry about it.