A common justification for behavior these days is, "it's only natural."
If the Christian perspective is correct, then human nature is corrupt. Just because a desire or behavior is natural doesn't necessarily make it acceptable or good.
On the other hand, if the atheistic naturalists are correct, then there is no such thing as the supernatural. If there's no such thing as the supernatural, then everything is natural. If everything is natural, then how can "it's only natural" be a justification? The justification is essentially, "it exists." Not much of a justification.
"It's only natural" is a ground for sympathy, but not for morality.