What conservative Catholics continue to call as "immoral" is the simple admission that human suffering is wrong and that we should do everything in our power to prevent it.
One of the most common reactions I hear from theists on atheism or even on freethought in general is a question on where the nonbelievers base their morality considering they do not hold themselves accountable to an absolute standard or have their actions confined by the threat of eternal fire.
"The atheist's morality of "do no harm" is actually much more complex than the theist's "10 commandments" when actually put into practice. By no means, however, does this make the atheist's morality inferior. Our morality is more nuanced, which many theists interpret as lacking principles, because they are unable to understand what isn't spelled out exactly for them in a simple list."