Professor Solum has an interesting post on the difficult distinction between sectarian and secular moral justifications for law. This is an interesting contrast with "natural law" theories—and most particularly with the intersection among law, science, and religious belief. The most obvious of these intersections is in abortion rights. There is no one right answer; there are, however, some clearly wrong answers, particularly at the extremes.
One easy case is the assertion that no restrictions on abortion of any kind are justified. Leaving aside the religious issues for the moment, that position is scientifically untenable. At some point during pregnancy, the fetus becomes capable of living outside the womb; sometimes with medical assistance, sometimes without. If Shakespeare can untimely rip a central figure in MacBeth from the womb, no one can claim that this is a new discovery.
The difficult cases, however, depend upon defining the beginning point of life. That is an inherently religious question based on current scientific knowledge. It is possible that some day science will be able to demonstrate to a scientific certainty that life begins at fertilization, or at the 32-cell stage, or at some later time. Our scientific knowledge is not at this time sufficient to answer these questions to scientific certainty. The questions can be answered to religious certainty; and that is the basis for most restrictive legislation concerning abortion.
This gets into the nasty question of particular motivations for particular legislation. It is a question that the courts have largely washed their hands of, on the ground that it is a political question. However, this is at best solipsism; the labelling of a particular issue as a "political question" is in itself a political act.
And this is where the concerns of authors and artists come in. Abortion is not the only area that includes an intersection among law, science, and religious belief. Obscenity is an obvious example, largely through what it excludes from its definition. A lurid description of sex between consenting adults—perhaps even a married couple consumating their marriage, which until relatively recently was a legal requirement!—can qualify as "obscene." A lurid, perhaps live, photograph of street-gang executions, however, does not. Obscenity is defined solely in terms of sex. (TV and cinema "ratings" aren't much better.) There is a strong, mixed political and religious motivation in obscenity laws and decisions.
That some "art" can be "harmful" to minors is beyond question. Exactly what art is harmful to a given minor, however, is a matter for parental control. Unless, of course, the Song of Solomon is to be treated exactly as was Fanny Hill.