While I agree with you, that free consent of the parties involved should be the ultimate determining factor, and the government should have no say in such intimate, consensual and personal decisions; another poignant point you made comes into play. Governmental intrusion into the most personal aspects of our lives, for purposes of micro management will be a rather drastic hurdle to any type of full-press legalization of incestuous cohabitation, much less marriage.
As the government becomes more involved with health-care related decisions, (as the Affordable Care Act's health insurance mandate exemplifies the first stages thereof)―preventative medicine will increasingly become the primary focus of both medical decisions and health-related regulations. The Center for Disease Control and Prevention will push for ever increasing regulations, not based upon individual choice, but on any amount of elevated risk. (The war on trans-fat comes to mind, or New York City's ban on large sugary soft-drinks). The goal will be keeping health-care decisions as cheap as possible, thereby keeping taxpayer subsidies, and premiums lower. (After all, bitterness is already spreading with relation to increased premium costs).
Consequently, I don't see the government taking the steps to legalize such relationships any time in the foreseeable future. And, if the government took such steps, the Center for Disease Control and Prevention, and an assortment of other “family protection” agencies would step in and regulate the legal status to such a degree, that it would wind up looking like the Texas Family Code provisions mentioned in my previous post. Alas, I'm neither condemning, nor condoning, such governmental actions, per se. Rather, I'm simply stating the reasons why I think it will not happen anytime soon, or, indeed, any time in the foreseeable future.
Now, with respect to keeping religious morality, or even morality in general out of the law; that task will prove difficult as well. I say this because of the fact that many aspects of the law, by its very nature, is related to such staunch beliefs. Take, for instance, the Criminal Justice System. Why does the government handle those issues, as opposed to the inflicted individuals? The answer boils down to the fact that the government acts, based on a moral wrong suffered by society, as a whole. I have often found this rather cantankerous, as it leaves open the question of “whose morality”, or “by what moral standards” we interpret such wrongs to have occurred.
Indeed, this creates a dangerous and rather dubious claim by the law. Yet, on the other hand, I see the need to prevent such actions from becoming increasingly wide spread. That being said, such arguments do seem to clash with the individual liberty, and the conscience of free will. The United States was founded on such notions. However, an additional hurdle presents itself to the legalization of such relationships, on the state level.
The Federal Government is limited to enumerated powers, as contained within the Constitution itself. State governments, on the other hand, have no such constraints, unless their actions conflict with federal law or the Constitution. In fact, states possess something known as “plenary” or absolute authority for law making. Part of the plenary powers is something known as the “general police power”. This “general police power” allows states to enact laws and regulations for the health, safety, morality, and general welfare of their people. That's a pretty wide swathe of protection.
Moreover, even if the parties seeking the consensual inter-related marriage or partnership brings a case all the way up to the Supreme Court, based on Equal Protection claims per the Fourteenth Amendment...I doubt it would get past any level of scrutiny. But, that point is moot anyway, because incestuous couples are not considered a protected class (such as race, gender, or ethnicity). Consequently, the case would be reviewed under the “Rational Basis” standard.
That is, the law or regulation against such relationships would not be deemed illegal, unless the government acted in an arbitrary or capricious manner in prohibiting or allowing such conduct. In other words, the burden of proof falls upon the complainant (or the aggrieved inter-related couple brining suit) to prove that the government had “no rational basis for enacting such laws or restrictions). That's where there's going to be a problem.
The court will look to the mere CHANCE that birth defects would result from this, or the mere CHANCE that this relationship might cause some sort of harm to the over all well being of the parties seeking to enter into a consensual relationship like this, and determine that it is rational for the government to make such regulations and decisions, thereby ruling against the couple.
Is that fair? Probably not. But, it is a standard that the court has used for countless generations. As a result, I simply don't see this issue being resolved in favor of such relationships, not unless the entire thinking of the governmental system shifts entirely.
With regard to the age of consent, this area is so nuanced in the law, and differs to such an extent from one piece of legislation to another...much less one state from another, or even at the Federal level, that it becomes difficult to register. The statutory rape laws don't even allow for the consideration or defense of consent. I never agreed with that aspect of the law, especially in cases where one party is aged 18 and another is say 16 or 17. (Or lower, depending upon the individual jurisdiction's age of consent).
However, in cases of minor marriage, the minor is allowed to seek permission from the court to allow a marriage, even if the parents refuse consent. The court must make a determination as to whether the minor is capable of “consent” despite the disability of minority (as it is called in law). There's a whole litany of factors which the courts sift through to make the determination as to whether the disability of minority should be removed.
To have the disability of minority removed, the minor will need to ask the court to appoint an “attorney ad-litem” for the minor, as per Texas Family Code §§ 31.002(b) and 31.004. The reason for this is, minors cannot even bring a claim to court on their own. Thus, the attorney ad-litem serves as what the law refers to as the minor's “next friend” (essentially the minor's legal representative).
The attorney ad-litem will then argue the merits for removing the disability of minority, which includes such things as: (1) the emotional and physical well-being of the minor; (2) the ability of the minor to make financial decisions for him or herself; (3) the current status of relationship between the minor and the minor's parents, etc. etc. This test is what is known as “The best interest of the child test”.
Again, this falls well short of allowing certain consensual decisions, but it does provide some leeway. However, with respect to incestuous relationships, I believe the major hurdle is going to be not only the perceived emotional and physical well-being of the minor, but also the effect that it has on the minor's relationships with their parents and/or guardians. Perhaps it could work, but again, I just don't see any court changing for that, like they have in regards to say same-sex marriage. (That area of the law is changing. And, the one nice thing about the American Justice system, being a common law, as opposed to a civil law system), is that Courts are empowered to change the status of the law, depending upon the changing needs of society. But, no legal system will ever be perfect. Still, just as with the case of polygamy, or even bigamy, I don't see the courts changing the prohibition against incestuous relationships anytime soon.