If only naturalism were unmistakably and irrefutably true! A burden would be lifted: no God, no soul, no personal survival of death, an assured exit from the wheel of becoming, no fear of being judged for one’s actions. One could have a good time with a good conscience, Hefner-style. (Or one could have a murderous time like a Saddam or a Stalin.) There would be no nagging sense that one’s self-indulgent behavior might exclude one from a greater good and a higher life. If this is all there is, one could rest easy like Nietzsche’s Last Man who has "his little pleasure for the day and his little pleasure for the night."
If one knew that one were just a complex physical system, one could blow one’s brains out, fully assured that that would be the end, thus implementing an idiosyncratic understanding of "When the going gets tough, the tough get going."
Some atheists psychologize theists thusly: "You believe out of a need for comforting illusions, illusions that pander to your petty ego by promising its perpetuation." But that table can be turned: "You atheists believe as you do so as to rest easy in this life with no demands upon you except the ones that you yourself impose." Psychologizers can be psychologized just as bullshitters can be bullshat – whence it follows that not much is to be expected from either procedure.
Am I perhaps falsely assuming that a naturalist must be a moral slacker, beholden to no moral demand? Does it follow that the naturalist cannot be an idealist, cannot live and sacrifice for high and choice-worthy ideals? Well, he can try to be an idealist, and many naturalists are idealists, and as a matter of plain fact many naturalists are morally decent people, and indeed some of them are morally better people than some anti-naturalists (some theists, for example) -- but what justification could these naturalists have for maintaining the ideals and holding the values that they do maintain and hold?
Where do these ideals come from and what validates them if, at ontological bottom, it is all just "atoms in the void"? And why ought we live up to them? Where does the oughtness, the deontic pull, if you will, come from? If ideals are mere projections, whether individually or collectively, then they have precisely no ontological backing that we are bound to take seriously.
The truth may be this. People who hold a naturalistic view and deny any purpose beyond the purposes that we individually and collectively project, and yet experience their lives as meaningful and purposeful, may simply not appreciate the practical consequences of their own theory. It may be that they have not existentially appropriated or properly internalized their theory. They don't appreciate that their doctrine implies that their lives are objectively meaningless, that their moral seriousness is misguided, that their values are without backing. They are running on the fumes of a moral tradition whose theoretical underpinning they have rejected.
If that is right, then their theory contradicts their practice, but since they either do not fully understand their theory, or do not try to live it, the contradiction remains hidden from them.