One example, I may think there is nothing morally wrong with taking mind altering substances, yet the person taking them might.
That is not a question of morality, but ethics. Ethics is cultural and subjective, morality, (when properly grounded in observed and analyzed factual data,) is not.
Also, just because people TREAT morality as subjective and mutable does not make it so. Granite becomes sand when you grind it into its constituent parts, but until it gets ground up, (through immense effort and/or time...) it is one of the hardest rocks on Earth. It is the same with morality: What most people CALL morality is no such thing: True Morality, as opposed to Ethics, is scientific in its observation of Situations, Conditions, Actions, and the Results arising from the same. Each part, like a grain of sand split from a granite boulder, is meaningless by itself, but taken as a coherent whole it is rock solid and stable. When one uses true morality ones conclusions are inevitable and unchanging, more stable than the very stones beneath our feet*.
*(Unless the very human race or the world(s) we inhabit are profoundly changed...)
On the topic of Incest; adopted siblings, step-siblings, or second/third cousins, or in laws, all count as incest. Leaving aside the age of consent argument, the US doesn't even have federal level laws, let alone uniform laws in the majority of states; in California I could fuck and marry my first cousin, and in New Jersey there are no penalties when both parties are 18 years of age or older. It is externally simplistic to deem Incest only one type: statutory rape, or that it does not has grey areas, much like underage kids fucking each other.
As I mentioned above, I am not discussing the bizare and rare edge cases such as step-siblings, god-siblings, adopted siblings, and brothers/sisters-in-law. These individuals, if their relationship is consensual, are outside the scope of this topic as far as I'm concerned. My points of argument are, have been, and shall remain the ruck and run of cases of incest, which are and have been primarily by older direct family members against those too young to understand the situation let alone give informed consent. Similarly I am not discussing the cases from royalty of lost cultures such as Egyptian Pharaohs and the Kings/Queens of the Hawaiian Islands: Though I will note that in both cases draconian culling programs were in place to deal with the consequences of inbreeding: programs that would not be permitted/condoned under the modern ethical standards of the areas in question.
My argument, rather, rests on a simple and irrefutable precept:
"The good of the many outweighs the good of the few, or the one."
No matter how many consenting individuals might be inconvenienced or forced appart by the moral fact that incest is overwhelmingly negative and therefore needs to be both ethnically and legally proscribed, the fact remains that these people are a vanishingly small minority. Therefore thier case can,
and must, yield to the needs of humanity as a whole. The
Ethical quandaries of the vanishingly rare edge cases of fully consensual incest I leave to wiser heads than mine. As I have already said, I am neither politician nor priest. Perhaps a different standard of ethics needs to be applied in those cases? Perhaps we need to develop a
better standard of ethics which is closer to True Morality? No matter the answers to these questions, facts remain facts, and the facts of this case are that the majority of cases of incest are also pedophilia. As such, the practice cannot be condoned.