Adult incest and the law

Father-adult daughter incest is in the news again; apparently incest among adults is not a crime in New Jersey (though incestuous marriages are void), and a father-daughter couple is planning on moving to New Jersey for that very reason. According to New York Magazine (Alexa Tsoulis-Reay), the daughter, who is now 18, grew up almost never seeing her father (except for weekends between ages 3 and 5). When she was 17, she got back in touch with him, and they quickly began a romantic and sexual relationship. (It's not clear what state this was in, so it's not clear whether the sex would have been criminal absent the incestuous component: three-quarters of states have the general age of consent set at 17 or younger.)

I thought this would be a good opportunity for me to ask again a few questions I asked some years ago about adult-adult incest (speaking specifically of parent-child, grandparent-grandchild, or brother-sister, and setting aside cousins and the like):

(1) Should it be illegal, and, if so, exactly why? Is it just because it's immoral? Because legalizing incest would, by making a future sexual relationship more speakable and legitimate, potentially affect the family relationship even while the child is underage (the view to which I tentatively incline)? Because it involves a heightened risk of birth defects (a view I'm skeptical about, given that we don't criminalize sex by carriers of genes that make serious hereditary disease much more likely than incest does)?

(2) Given Lawrence v. Texas—and similar pre-Lawrence decisions in several states, applying their state constitutions—what exactly is the basis for outlawing adult incest? (Imagine, for instance, a relationship such as the one described earlier in the post, but one in which there was no sex—and even no preparations for sex—until after age 18.) Is it that bans on gay sex are irrational but bans on adult incest are rational, and rationality is all that's required for regulations of adult sex?

Is it that bans on gay sex don't pass strict scrutiny (or some such demanding test) but bans on adult incest do? Is it that Lawrence rested on the fact that bans on gay sex largely foreclose all personally meaningful sexual relationships for those who are purely homosexual in orientation, whereas incest bans only foreclose a few possible sexual partners? (For court cases on this, see here (stepfather-stepdaughter) and here (brother-sister).)

(3) If adult incest is indeed criminalized, what should the penalties be (assuming lack of further aggravating circumstances, such as force, strong evidence of grooming for future adult incest during childhood, and so on)? Should the penalty be relatively light, on the theory that only consenting adults were involved (much as the penalty for prostitution is relatively light)? Should it be very grave, on the theory that it's important to send a firm, unambiguous message that such behavior is wrong, or perhaps on the theory that one party is likely to have seriously harmed the other even though the other consented?

(4) Should all this apply to adult brother-sister incest, or are the arguments chiefly limited to what one sees as the likely special emotional control that parents might have over children?

UPDATE: By the way, my former coauthor, Brett McDonnell, has an interesting article on the constitutional question, Is Incest Next?.