Adultery and murder have happened in just about every human society in the history of time but nobody thinks we shouldn't bother trying to suppress those impulses.
Urbanization isn't unnatural and it agrees with our tribal instincts.
Look at Tokyo: extremely high density with extremely small social unrest. Paris, now a collection of different tribes, can't say the same.
On another note, I think Japan ends up with kind of a distorted picture since so much of the crime is associated with organized criminal groups with ties to legitimate business and government operating with a degree of openness that I think would surprise most Western observers. Dubro's Yakuza book is a great primer on the topic.
I'm not claiming that. I am claiming that multiculturalism plays a role, which many people refuse to acknowledge.
And for murder, that can be a bit of a tautology as socially unacceptable killing is murder and anything else isn't considered on the same level as murder. But if we consider killing that isn't considered murder and if you consider forms of killing that, if not legal, are punished lighter than murder, it becomes clear that our society does allow some forms of killing others.
It does if there are enough nitpicks showing that natural behaviors are becoming more accepted because they are natural. That isn't to say they are acceptable, but they are more accepted. And this doesn't have to be true of all behaviors, because there are many factors at play. It is even possible there could be a trend where behaviors are becoming more acceptable because they are natural even if all behaviors were becoming less acceptable, if there was another stronger force acting to make all behaviors less acceptable.
But if we want to get to the actual argument first presented, we would have to realize that comparing it to murder, adultery, or sexual assault is a strawman to begin with. The OP wasn't defending going around and murdering people who didn't look like you. They were saying that millions of years of evolution will influence our behaviors in small ways. It would be compared to other preferences, such as preferences in intimate partners. So, is wanting to have a specific gender, race, etc. in an intimate partner wrong? Is it socially unacceptable?
It's a factor way too often ignored with this issue: If "hurting people's feelings" will become a crime, then we will see no end of regulating "undesired behaviors" because people get their feelings hurt in the most subjective and unintended ways [0].
So much about "obvious examples of racism".
[0] http://thedmonline.com/greek-life-retreat-ends-abruptly-bias...
More to the point, though, racism is an obvious manifestation of the kind of tribalism I think we are talking about and I don't think it's really controversial to argue that it is harmful at least to the marginalized groups. I'd personally even go further and say it harms all parties involved.