Do _you_ want to be on the side that argues that our modern interpretations of age of consent may be a fad? Or that one day science may provide incontrovertible evidence that racism is entirely rational and appropriate?
Arguing these is pointless, because they are violate dogmas that underpin our modern ideology. They are not up for argument. If another ideology suggests that racism is actually a-ok, then we consider it clearly flawed, because dogmatically, ideologies that come to that conclusion must be flawed. And not just flawed, because these fundamental dogmas are what shapes our notions of "good" and "evil". Not adhering to these dogmas makes you evil.
I must add, this is not a fallacy of some sort. If I were thrown into another society where everyone approves of racism, I am still attached to my personal dogmas. I would consider such a society unjust and warped. As a matter of fact, there's plenty of personal dogmas our current society violates, and I consider our current society and its ideology unjust and flawed.
Which raises another point, which is that our society does not have a monolithical ideology. It has an emergent ideology that arises from common agreement, but there's plenty of people in our society these days who disagree with several of the dogmas. And that's what makes them "bad" or "evil".
The only way to argue someone out of a personal dogma is to convince them that whatever your dogmas are is reflective of absolute good, evil, and/or truth. This is the subject of what is likely one of the oldest philosophical debates and has spawned several religions.