Yes, this is why I put "objective" in quotes and compared it to a brake. Traditions do evolve over time, and even text-based religions see changes in practice and interpretation. But at the end of the day, people can make powerful moral arguments based on a long-existing foundation. That slows down the pace of change.
> Accepting the truth of the matter isn't going to cause society to collapse into barbarism and hedonism.
Unmooring social norms and mores from traditional foundations has indeed historically resulted in extremely violent periods. I am not assigning a moral value to this violence, but I am glad I didn't live through it.
> And from an atheist point of view it's obviously possible to accept a moral framework in a morally relative universe because that is what everyone, including religious people, are actually doing
I am not arguing that people do not accept moral frameworks (as social animals we do whether we like it or not), but that typically, atheists, like most people, do not acknowledge that moral frameworks are relative, and jump at every chance to sanction people from different cultures for violations of their moral code. Of course, some atheists might avoid making a moral argument for this very reason! We can always acknowledge that things are simply a matter of our preferences and still advocate for them.
But typically, you instead get vague, waffley treatments about how we have made new discoveries in human rights that, for example, require you to permit gay marriage or face the imperial might of the US. At the end of the day, exactly because this sort of morality lacks persuasive force, it relies more on various sanctions. Meanwhile, theists of varying stripes can fall back on the very powerful argument of "God said so", and trust that he will reward you accordingly for disobeying his dictates. Of course, it's not always the case that atheistic moral realists will bomb you into submission, just as it is not always the case that theistic moral realists won't decide they're the scourge of God. But in 2021, that's how it typically plays out. In short, this particular brand of moral realist seems to be the one presenting to the most danger to the rest of us.
> The Bible says slaves should obey their masters, that women are as far beneath men as men are beneath God, and that homosexuality is an abomination worthy of death
I left off this until the end of the post because I don't feel particularly comfortable defending Christian thought, but I think these are misleading characterizations of what they say.
The Bible, including the New Testament, does say that slaves should obey their masters. But the same individual (Paul) writes that there is neither slave nor free among Christians. Men are equal before God, though in the world they are placed in hierarchies - this is a common theme. Paul preaches for servants to obey masters for the same reason that Christians are instructed to bless their enemies and do good to them that hate them and persecute them - because by so doing they show their faith in God. I don't think this is itself morally objectionable even from our perspective. 'Turn the other cheek' is still considered the moral high ground by most people, and this is coming from the same place.
Similarly, although homosexual behavior is definitely treated as an abominable sin, it is not treated as worthy of death moreso than other sins, including heterosexual adultery. Although Christians are instructed to cast out other Christians who continue practicing sinful ways, they are otherwise told not to judge. Indeed there is a specific passage where Jesus stops an execution. Of course, once Christians were in actual power, they did often legally sanction sins beyond those we might consider civically useful, but this rarely occurred in a way worth mentioning - homosexuality and adultery remained common and were rarely carcerally punished for most of their 2000 years in power. They certainly were sometimes, though!
I am not aware of any part of the Bible that takes such an extreme view on women. The most extreme stance I know of is taken in Timothy 1, where Paul claims that women should not teach and that men have authority over women because Adam was created first and was not deceived. But nowhere can I find a statement as extreme as yours. It would not surprise me if it were the case many or most practicing Christians still believed these things, although obviously the first one has become moot.