Excluding gay people from marriage is about discrimination, hatred and bigotry.
It's not about changing other cultures, it's about allowing those who do not conform said culture to live their lives in freedom and equality.
This is the typical bullshit argument used against gay marriage: that it somehow negatively affects the existing cultural traditions. It doesn't.
Indeed, in cases where slavery is primarily endogenous (say, ancient Greece or Rome, or even Viking-age Scandinavia), slaves tend to have some legal rights, and some freedoms because it functions as a bit of a safety net, and there are political incentives to free slaves. For example in those three cultures (Greece/Rome/Viking-age Scandinavia), freed slaves formed important sources of political and economic support. Dred Scott v. Sanford ensured in he US that such a solution could not exist here.
So slavery in the new world was uniquely evil and oppressive. One can defend many other systems of slavery in the world throughout history, but certainly not that one. Indeed the only second-person perspective defence that was made was that we were so much better than the Africans that it was better to be a slave on a plantation in Virginia than free in Senegal. That same arrogance is, once again, behind this today.
Heck, I won't even defend the system of prison labor/slavery we have in the US today because I think that's beyond defence. But I won't just say "we're labelling it slavery and therefore it's evil."
So in New Guinea there is an indigenous tribe called the Sambia. In this tribe you can't become a man unless you go through a coming of age ceremony where you are given a nosebleed by having a tooth of a wild animal shoved in your nose (this is to drain feminine influences, and remove childhood), and where you are required to fellate a tribal elder. Of course this happens at an age roughly similar to adolescence.
Now obviously if this happened in NYC, we'd call this sexual predation, and try to send people to jail for life. But are you saying that we need to insure that if indigenous children don't want to go through such rituals that they have a right to be left alone?
What about a sect that moves to an isolated island and comes up with the tradition that girls first sexual act has to be with the sect leader (such things actually happened)? Why should they be treated differently than some indigenous tribe?
That's not always true with endogenous slavery though. Very often times people end up as slaves, selling themselves into slavery in order to avoid starvation. The constant turnover between free and slave in these systems curbs the worst abuses. For example, in Rome, it was illegal for a father to sell a son into slavery more than three times. After that, I suppose, it was the father's turn......
in these systems typically slaves have some level of personal autonomy, and there is pressure to free them. In Rome for example, freed slaves were adopted into the extended family of the former master, thus providing legal, political, and economic support for the great Senatorial families. This meant paradoxically that slavery could actually be a means of social advancement.
So I wouldn't say that was always true. In many of these societies, then, prisoners of war end up being a secondary source of slaves but they end up with the protections of the debt-slaves, again with the pressure to free.
One more thing that surprised me about the Roman example was this: Slaves could join professional associations (sort of proto-Guilds) and these associations usually handled burials of their members and the like. Many of these organizations actually had rules adopted for what happens if a member was a slave and the master refuses to delver the body for burial.
So slavery is a complex thing, and a lot of blanket statements don't necessarily work. Yes, a lot of slaves in Greece, Rome, etc. did choose to be slaves because it was better than the alternative, which was being destitute.
Also keep in mind that in most of these places, there is no strong central prosecutorial authority. Your family is supposed to prosecute crimes on your behalf. So if you were in ancient Rome, destitute and without extended family, you really were better off selling yourself into slavery.
As for the indigenous tribe, at least I am not sure that we shouldn't interfere if those kids don't want to go through those rituals.
A sense of humility, that our culture doesn't necessarily have more answers than theirs, and that we don't really understand what we are messing with.
I don't think that counts as "voluntarily becoming a slave". Obviously it is complicated (I like to call modern workers "wage slaves" occasionally - obviously they tend to work because they must eat, too). But for example where I live the law puts some limits to exploitation, a basic fairness built into contracts. It would be illegal to make somebody a lifelong slave in exchange for an apple, just because they were starving at some point and needed the apple or die.
"A sense of humility, that our culture doesn't necessarily have more answers than theirs, and that we don't really understand what we are messing with."
But where do you draw the line? I think at some point you need to stand up for your values. If you are wrong and the others are right, but you are stronger, it is of course bad luck. But how do you prevent that? I guess one of the cornerstones of western societies is protecting children.
Maybe as a compromise that tribe could raise the age the kids need to have before they consent to the initiation rite...
Again the question, what about the pedophile sect?
Or another example: what about female circumcision? Should we accept it, because it is ingrained in some cultures?
I think that misunderstands the nature of endogenous slavery at least in the Roman and Scandinavian models. People aren't selling themselves into slavery for an apple. They are selling themselves into slavery for long-term subsistence and possibly an opportunity to prove themselves worthy of being freed (and thus adopted into a more powerful family). It's not really that bad of a deal really, compared with little legal protection, no income, and food insecurity.
Additionally political power comes with both buying slaves and freeing them, so being an attractive "business partner" in this regard isn't so bad a thing.
But where do you draw the line? I think at some point you need to stand up for your values. If you are wrong and the others are right, but you are stronger, it is of course bad luck. But how do you prevent that? I guess one of the cornerstones of western societies is protecting children.
Two places.
I think that direct action is appropriate only when directed at one's own culture. I don't think I have a right to insist that Indonesia recognize Jewish weddings. I do think I have a right and an obligation to insist that US corporations in Indonesia act in accordance with my ethics, to the best of my ability.
On general advocacy though, I draw the line at specificity:
It's ok to critique a specific culture and practice, looking at specific harms and trying to bring awareness as to social costs and injustice.
It's not ok to simply say "you must do things the way my American group thinks would be pretty cool if it happened in America but we haven't got there yet."