From an evolutionary perspective, it makes complete sense that ethics and morals will also evolve.
Personally, from my JW perspective of absolutes with everything being black/white, the concept of ethics and morals changing becomes an earth-shattering crisis. How do we take bronze age laws of the OT and pretend that it was a good thing that a girl had to marry her rapist... as his punishment for the crime? When issues of right and wrong came up if field serve-us, it was mental gymnastics to try and fit some cherry-picked scriptures to conform to what is clearly acceptable/unacceptable in modern society.
As far as slavery, it ended, not because there was "new light" in the bible, it was the evolution of economics, recognition of the abuses, and plenty of other reasons. Slavery still exists, but it's gone from being the accepted standard to something considered evil.
As far as "spreading genes", it used to be advantageous to have many children from many mates, regardless of whether you are debating religion or evolution. Life used to be full of peril, disease, and many threats that made abundance an advantage to survival. With the evolution of society, it became more advantageous to have few children, well educated and also marrying well, and with strong emotional ties in the family. An evolutionary point of view wouldn't just say to spread more genes by random breeding. It would be about assuring the survival and prosperity of the children carrying my genes. Evolution, and common sense, would dictate that I'd be better off with a couple healthy, smart kids that I'm very close to, rather than a bunch of strangers that are struggling for their own survival.
As far as women's rights, that's gone back and forth through history. The modern trend of better educated women with more rights, authority, and options, has resulted in greater productivity, longevity, improved infant survival, etc.
As far as gay rights, why stone to death that lesbian aunt or gay cousin? From an evolutionary point of view, and maybe also from my point of view, they will give my kids gifts and affection and support for their education and success. Gay rights might not be supported by biblical religious "morality", but from an evolutionary POV, let them have their rights. Let them marry/divorce/live in peace. It doesn't take away any of my rights.
Now that my JW perspective has been replaced by "worldly" ideas of evolution and atheism, I don't see my ethics as being weak or unreliable just because I'm not afraid to debate or change. I see them as something that will evolve. I don't want my morals to be dictated by ancient and questionable standards set by the bible. Perhaps others see it as meaningless tyranny by the majority. The fact is that the issues challenged as right/wrong often begin with a minority, followed by increasing understanding and support, then it can become the view of the majority... sometimes not.