The post below, and the discussion in the comments, leads me to ask: Precisely why might it make sense to outlaw sex between stepparents and adult stepchildren? I don't want to ask whether such a ban should be constitutional, or whether criminal penalties are ultimately a good idea. (One might conclude, for instance, that outlawing such sex may make sense, but that on balance it makes more sense not to have the legal system try to investigate and criminally punish such things.)

In the course of describing the rational basis for the law, the court gives what might be an answer to my question:

Ohio has a tradition of acknowledging the "importance of maintaining the family unit." A sexual relationship between a parent and child or a stepparent and stepchild is especially destructive to the family unit. R.C. 2907.03(A)(5) was designed to protect the family unit by criminalizing incest in Ohio. Stepchildren and adopted children have been included as possible victims of the crime of incest because society is concerned with the integrity of the family, including step and adoptive relationships as well as blood relationships, and sexual activity is equally disruptive, whatever the makeup of the family. As the "traditional family unit has become less and less traditional, * * * the legislature wisely recognized that the parental role can be assumed by persons other than biological parents, and that sexual conduct by someone assuming that role can be just as damaging to a child." This reasoning applies not only to minor children, but to adult children as well. Moreover, parents do not cease being parents -- whether natural parents, stepparents, or adoptive parents -- when their minor child reaches the age of majority.

Accordingly, as applied in this case, R.C. 2907.03(A)(5) bears a rational relationship to the legitimate state interest in protecting the family, because it reasonably advances its goal of protection of the family unit from the destructive influence of sexual relationships between parents or stepparents and their children or stepchildren. If Lowe divorced his wife and no longer was a stepparent to his wife's daughter, the stepparent-stepchild relationship would be dissolved. The statute would no longer apply in that case.

But this strikes me as pretty abstract -- how exactly is the "family unit" threatened, and what exactly does "the integrity of the family" mean? Even if you're confident that there's some harm here, it helps to know concretely what the harm is, since that will help us understand what boundaries the incest rule should have.

For instance, is the threat simply the threat of dissolution of the stepfather's and mother's marriage? If so, should the law apply when the mother is dead (in which case I take it the stepfather would still be the stepfather) -- and why should the law apply even when the mother is alive, given that other forms of adultery are no longer criminally punished?

Is it the threat of damage to the stepfather's/stepdaughter's familial relationship, in the event that the stepfather/stepdaughter break up their romantic relationship? If so, why shouldn't the law apply even if the father divorces the mother? (It may be that it literally doesn't apply, given its text, depending on how you read "stepparent," but maybe that just means it should be amended.)

Is it the threat of damage to the mother/daughter relationship? If so, then again why shouldn't the law apply even if the father divorces the mother to sleep with the daughter?

Here's my very tentative thinking on the matter. First, I don't think that the "abuse of authority" argument raised by some commenters is terribly powerful here, if the argument presupposes that the stepfather is using his authority or even his father figure status coercively. Parents, including stepparents who have raised the child for many years, have notoriously little ability to control their adult children; at most, they'll know which psychological buttons to push, but that doesn't seem to help that much. Perhaps in unusual cases stepparents would have some sort of psychological power over their adult children, and maybe cases of stepparent-stepchild incest disproportionately involve those unusual cases. But I'm not sure why we should assume this, and why we should think such psychological power is that much greater than in many other situations where the relationship isn't criminalized. (Even boss-subordinate relationships, for instance, are almost never criminal, and aren't even by themselves cause for civil liability, though they may carry the risk of civil liability if the jury finds some threat of employment retaliation underlying the relationship.)

Second, I don't think that the concern can just be that the stepfather is breaking up his marriage with the mother (that's no different from normal adultery, which, while bad, is generally not treated as a crime) or even jeopardizing the mother's relationship with the daughter (though I might be mistaken on that point, since stepparent-stepchild incest is unusually likely to jeopardize the parent-child relationship).

Third, it seems to me that the problem with a stepparent-stepchild sexual relationship is not that it jeopardizes the relationship between this stepparent and this stepchild. That's a risk that this stepparent and stepchild can be expected to assess for themselves.

Rather, it seems to me that the most sensible argument for banning stepparent-stepchild sexual relationships -- and perhaps other familiar sexual relationships -- is to diminish (as much as possible) the risk that normalization of sexual attraction in stepparent/adult-stepchild relationships will color others' stepparent-stepchild relationships, including ones that involve minor stepchildren.

When a stepfather looks fondly at his stepdaughter, we want her thinking "he loves me [as a daughter] and thinks I'm beautiful," rather than "he lusts after me." When a mother looks at her husband's looking fondly at his stepdaughter, we want her thinking "he loves her [as a daughter], which is what I want," rather than "he lusts after her." When the stepfather is thinking about his stepdaughter, we want him to banish lustful thoughts as much as possible, rather than thinking about how he might arrange things so that he can sleep with her -- even if he waits until she's 18.

And this is an especially compelling problem for stepfather/stepchild relationships because, as best I can tell, there really is nothing biologically unnatural about them; it's naturally for adult males to be sexually attracted to young postpubescent females with whom they're not genetically related. (Perhaps some innate psychological taboos may undermine that when the adult male has raised the girl from early childhood, but many stepparent/stepchild relationships start when the stepchild is no longer a very young child.) There is thus special social need to desexualize these relationships as much as possible. Of course, we don't want incest to be so unthinkable that it becomes unreportable if it happens nonconsensually, but we do want it to be thought of as little as possible unless there's really solid evidence of its being imminent.

If I'm right, then the behavior control here is a means of affecting other people's understanding and expectation of how people behave and think (as well as how they should behave). I should say this is not necessarily a pretty story: Rather than relying on not-very-far-from-libertarian concerns about coercion (even of adults) or violation of promises (such as the marriage vow), this argument for not allowing stepparent/adult-stepchild incest is a form of thought control. Yet it seems to me to respond to a more serious problem than the other arguments, because it suggests that control of this incest is needed not just to protect a few adults from emotionally damaging decisions, but to protect millions of adults and children from the harm that may stem when their stepparent-stepchild relationships become perceived as potentially sexual even when they themselves firmly resolve not to allow the relationships to actually become sexual.

Moreover, if this story is right, then this suggests that the incest ban should apply even if the stepfather and stepmother divorce, and even if the stepfather is a widower. Any stepparent/stepchild sexual relationships risk normalizing such relationships and thus potentially sexualizing people's attitudes towards such relationships more broadly. And it also suggests that adult stepsibling sexual relationships may likewise be harmful in a way similar to what I describe (though again I should note that this doesn't necessarily mean they should be criminalized).

I may well be wrong on all this. This isn't my core area of expertise, I haven't thought about it a great deal, and thankfully I don't even have personal experience with it (lacking either stepparents or stepchildren). But I thought I'd mention this, and see what others have to say.