I don't think we arrived here because of rigorous logic, but because of changing conditions (rejection of feudalistic marriage, rise of eugenics, alienation of people from their communities, etc).

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It's why I highlighted the fact that marriage between cousins isn't just a white phenomenon, a lot of colonized peoples have cultures of endogamy and arranged marriages and to blanketly assert that there is something wrong with those cultures is chauvinistic.

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Any cultural practice can be good or bad

I'm not disagreeing with these statements in a purely academic sense; as far as I'm aware, Marxism and dialectical materialism doesn't really vibe well with morality and it doesn't seem like the debate between moral objectivism and moral relativism is something we deeply concern ourselves with, or perhaps we could say that there's some "proletarian morality" that is "superior" to "bourgeois morality"; idk, I'd have to read more and refresh myself about the philosophy of Marxism here.

What I'm trying to get at is how we approach this in a more colloquial sense. And I'm talking about the general style of argument here now, not so much the specific question of "Is fucking your cousin good or bad." Because when I put my Marxist hat on, I could say the points above quite comfortably - there is no moral objectivism! Conditions govern what people believe and why, and there is no bestowed moral code, from a deity or particularly wise person or otherwise, that governs humanity in all situations, even if those moral codes were very complicated. Like, not just "You shouldn't steal." but "You shouldn't steal, unless it's for survival, or unless it's from a person or company that would not even notice it missing while you would greatly benefit, or--" etc.

The trouble begins when you say that moral objectivism is false and then somebody tells you that they just read an article about misogyny, and, wow, that really sucks, doesn't it? Well... does it? Do/should you launch into a historical analysis of misogyny and its foundations and oppression, and how certain countries in the past and present have had misogynistic policies and a culture of oppressing women without really going into whether it's moral to be a misogynist, or do/should you say "Yeah, misogyny fucking sucks and is never okay, and all misogynists should be punished?"

Here's something else: a couple months ago, a user here wrote up a piece (featuring a quote from yours truly!) on Latin America and whether it should be regarded as Western. This did get me thinking about a potential situation that I could one day experience, as a British person. Imagine that I went out in public and went on a long rant to a friend on how the British Empire fucking sucks, it was awful, it killed millions of people, the culture is/was bad, it involves racist worldviews, and so on. Imagine an Asian immigrant from Hong Kong who moved to the UK who identifies strongly with British culture overheard me and said "No, the British Empire was great. It spread law and order throughout the world, it developed countries - including Hong Kong, where I grew up! - and was a massive force for good in the world. By presenting this overly negative argument, you're being a chauvinist." What should my hypothetical response be? She's presenting an argument from the standpoint of moral relativism, saying that it would be chauvinistic for a white British male to inform a female Asian immigrant what they should find moral. Does this mean that the British Empire truly cannot be objectively evaluated as one of the worst regimes the world has ever seen?

So the problem I fundamentally have with the argument here about whether fucking cousins is morally good or bad isn't so much a debate of the cold hard facts of cousin-fucking throughout history and how many countries and cultures have done it without being really that negatively impacted by it and so on, and whether, on those grounds, it would be wrong to say that "you shouldn't fuck your cousin" because that's unscientific or even chauvinist; it's how far this argument can actually extend. Let's say that we do ultimately come to the conclusion that despite fucking your cousin being generally seen as taboo in Western societies, this doesn't matter because Western societies can suck our collective dicks and having a romantic relationship with your cousin has been a part of many places for millennia - we cannot prescribe arbitrary moral laws on other people just because we think that something is icky or taboo. It has to be rooted in science!

Okay, what about a little more questionable topic, like stepsiblings? And so on through moral issues of increasing complexity that may not have clear, amoral, scientific answers? If we're dialectical materialists then do we reject moral objectivism (or, hell, even morality entirely) even in casual conversation, or can we say "Killing slaveowners is fucking awesome whether it's in 5000 BC, the 1800s, or today, and every single place on Earth, regardless of culture?" Because I sure want to keep saying that. Consent is a moral issue that I would regard as extremely important, but I could easily imagine that there have been cultures and civilizations before that have regarded the consent of one group or another as an arbitrary moral requirement that they would consider as much as they would consider whether having a romantic relationship with your cousin is a good or bad thing - that is, not at all. I think that they would be very morally and culturally wrong. They might not understand what the big deal is. I think the consent is an important moral issue because I think harming people is generally wrong, with certain exceptions (slaveowners, for one). I can't really scientifically "prove" that harming people is wrong - in fact, the largest, richest, and most powerful empires on the planet got there explicitly by harming a shitload of people. It's a morality, even a cultural norm, that I am asserting.