A little translation is in order here. Glucksberg is not, in fact, the leading modern case on substantive due process and implied fundamental rights. That would be Griswold or Eisenstadt (as reinterpreted by later courts), or (shudder!) Roe, or Casey or Lawrence, or now, Obergefell.
Rather, Glucksberg is the case that people opposed to implied fundamental rights wish were the leading case and regularly cite as if it were the leading case that applied to every such question.
Chief Justice Rehnquist--no fan of implied fundamental rights himself--wrote Glucksberg in 1997 precisely to lay down a marker so that federal judges would stop trying to imply fundamental rights.
He didn't succeed.
Alas, the Court hasn't taken Glucksberg very seriously since it was decided. Not in Lawrence. Not in Windsor, and certainly not in Obergefell.
Here's what Justice Kennedy has to say about it:
Another way of putting this is the the debate between Justice Brennan and Justice Scalia in Michael H. v. Gerald D. is now over, and Justice Brennan won. Justice Scalia (joined by Chief Justice Rehnquist in a famous footnote) argued that traditions of liberty should be construed as narrowly as possible--"the most specific level at which a relevant tradition protecting, or denying protection to, the asserted right can be identified." Justice Brennan argued that our traditions should be construed abstractly and broadly, so that some existing practices might actually be inconsistent with our traditions properly understood in their best light. At the time, Scalia's account had real difficulties accounting for the Court's jurisprudence up to that point; it wasn't really consistent with Griswold or Eisenstadt, much less Roe. But that was fine with Scalia-- he really didn't think much of those cases anyway, and he didn't want to extend them any further.Objecting that this does not reflect an appropriate framing of the issue, the respondents refer to Washington v. Glucksberg, 521 U.S. 702, 721 (1997), which called for a “ ‘careful description’ “ of fundamental rights. They assert the petitioners do not seek to exercise the right to marry but rather a new and nonexistent “right to same-sex marriage.” Brief for Respondent in No. 14–556, p. 8. Glucksberg did insist that liberty under the Due Process Clause must be defined in a most circumscribed manner, with central reference to specific historical practices. Yet while that approach may have been appropriate for the asserted right there involved (physician-assisted suicide), it is inconsistent with the approach this Court has used in discussing other fundamental rights, including marriage and intimacy. Loving did not ask about a “right to interracial marriage”; Turner did not ask about a “right of inmates to marry”; and Zablocki did not ask about a “right of fathers with unpaid child support duties to marry.” Rather, each case inquired about the right to marry in its comprehensive sense, asking if there was a sufficient justification for excluding the relevant class from the right. See also Glucksberg, 521 U.S., at 752–773 (Souter, J., concurring in judgment); id., at 789–792 (BREYER, J., concurring in judgments).That principle applies here. If rights were defined by who exercised them in the past, then received practices could serve as their own continued justification and new groups could not invoke rights once denied. This Court has rejected that approach, both with respect to the right to marry and the rights of gays and lesbians. See Loving 388 U.S., at 12; Lawrence, 539 U.S., at 566–567.
Justice Brennan's view of tradition is consistent with the second Justice Harlan's, who famously wrote of "the balance struck by this country, having regard to what history teaches are the traditions from which it developed as well as the traditions from which it broke. That tradition is a living thing." That is, Justice Harlan emphasized that sometimes following tradition means rejecting some traditional practices or assumptions that we now believe are no longer consistent with our traditions considered in their best light. Our traditions are living because they change over time, for example, by becoming more inclusive.
The Chief Justice quotes part of this famous passage from Poe v. Ullman in his dissent. But he leaves out the words "as well as the traditions from which it broke. That tradition is a living thing." In doing so, he misunderstands what Harlan was saying about the right to use contraceptives.
In Poe, Justice Harlan was not making an argument for judicial restraint. He was arguing that sometimes the best way to read our traditions is as evolving, and therefore as rejecting some previous practices that we previously regarded as part of our traditions. In like fashion, Justice Kennedy argued that the best way to understand our traditions of marriage is as evolving-- we have gradually been moving toward a view of marriage as a bond of commitment between two equals, which is as compatible with same-sex marriage as it is with opposite-sex marriage. Considered in their best light, same-sex marriage is fully consistent with our traditions of liberty, not opposed to them.
Justice Harlan's approach in Poe v. Ullman is not consistent with the narrow reading of tradition in Glucksberg and Michael H. It is consistent with the Court's decisions in Romer, Lawrence, Windsor, and Obergefell.
After Obergefell, Glucksberg is clearly no longer the leading case on substantive due process, if it ever was. It is the leading case on assisted suicide. It will have little impact elsewhere until some future majority the Justices wants to reject a different claim of implied fundamental rights. Then, no doubt, Glucksberg will be trotted out again. But it should be clear that by now it is only a makeweight, and it does not announce a standard methodology for evaluating claims of constitutional liberty.