I think this is a fundamental misunderstanding of how moral systems work, one that is perpetuated by the church.
In Christianity, there is a specific "source" of morality, one that's (supposedly) infallible and unquestionable. This leads to problems when the infallible-unquestionable source of morality endorses things like slavery [1]: if it's unquestionable and infallible, how can your moral system be adapted to address this shortcoming? (This is usually handwaved away by theists by saying something like "oh that's the old covenant", or "that was a different god", which uses strange internal doctrinal shenanigans to somehow justify this supreme perfect being's obvious moral shortcommings, and looks to everyone outside the religion like someone just playing the cup game.)
Outside of religion, there is no single "source" of morality. There's a codified system of laws that our societies adhere to, certainly, but there's plenty of examples of how the laws do not codify morality (e.g. I am not required to be even a minimally-decent Samaritan to anyone I encounter on the street). Morality instead is something that an individual has to build for themselves, as a framework for decisionmaking in the wider world: how do you choose between your available actions? what are your guiding principals?
Some decide that, for various reasons, they cannot support industrial animal agriculture, and turn vegan. Some decide they fundamentally disagree with the structure of our police forces. Some turn pacifist--but all come to these conclusions through their reasoning. They absolutely can (and do) borrow reasoning from other sources, but it's impossible for it to be 'imposed' on them because there is no 'higher power' that has the absolute moral right.
This is something that's frustrated me when trying to engage with religious people on other moral issues; a few months ago I was talking to my mom about the concept of wealth redistribution, as to me it seemed incredibly immoral that people like Jeff Bezos can have an insane amount of wealth while we still have children in our schools who can't afford lunch or library books or are even homeless. My mother (Christian) couldn't get around the idea that she had no right to impose her perceived morality on another: that it would be wrong to support laws requiring the redistribution of wealth. She had similar issues with LGBT rights: she could see the harm caused to LGBT people, and did not want to participate in that harm, but struggled to support them because she felt like codifying moral support for something, which she didn't have the power to do.
This removal of one's own ability to develop and test morals is part of what disturbs me greatly about the church. It makes it very difficult to demonstrate when people make mistakes, how they have harmed other people, and how they might go about avoiding that in the future.
[1]: https://www.biblegateway.com/passage/?search=Exodus+21&versi...