Identity and liberation
The best-known example of this is the shift from primarily-religious to primarily-national identity in the Western world over the last few centuries. During the Dark Ages when European culture was dominated by fervent Christianity, religion determined identity -- "the other" was the Jew, the Muslim, and whatever pagans still survived, or were suspected to survive, in odd corners of the domain everyone called "Christendom". Even within that domain, the bloodiest wars were waged not between nations but against "heretics" like the Albigenses or rival sects, such as the Thirty Years War between Catholic and Protestant Christianity. Over time this shifted to a sense of ethnic / national identity based on common language, culture, and history (granted that these were often somewhat artificial, contrived by governments to justify existing or desired borders), and in recent centuries Europeans have fought as fiercely for la patrie or das Vaterland as their ancestors once fought for this or that True Church against unbelievers.
In the last few decades we've seen the emergence in the West of a quite novel form of group identity -- sexual orientation. Gay people have become, and are now largely viewed as, a distinct community like an ethnic minority, with its own culture, flag, self-pride, and "national interests". This concept would have struck anyone before the 20th century as extremely strange, but it's easy to see why it happened. The early gay liberation movement faced a formidable task in a society where homosexuality was illegal, widely reviled, and condemned as sinful by the dominant religion. And it emerged at a time when the black civil rights movement was beginning to make progress against the equally-daunting forces of entrenched and murderous racism. It's only natural that the gay movement, consciously or not, modeled itself on another great struggle then under way.
With victory in sight, however, it's appropriate and likely necessary to take another look at our premises. The re-purposing of homosexual orientation as the basis for a distinct group identity may have contributed to the startling success of gay liberation over the last couple of decades, but it's hard to see how anything analogous could work in coming phases of the struggle such as, say, decriminalization of marijuana or sex work. We can't let ourselves get mentally locked into just one model of how to fight.
Indeed, the very concept of a quasi-ethnic gay identity would not exist outside the context of repression. In the Classical civilizations, homosexuality in males was not a "sin" or even a particularly salient trait. It was noticed that some men were attracted mainly to females, some mainly to males, and some equally to either, but none of these preferences was considered a marker of an identity or a deviation from a social norm -- and as far as I know, neither ancient Greek nor Persian nor Latin had a noun equivalent to "a homosexual" or "a gay person" in the sense that we use such terms today. It was something you liked, not who you were. Ethnic identity, on the other hand, was very strong even back then, as any overview of Greek attitudes about "barbarians" will illustrate.
One could argue that if our dominant religion damned left-handedness as a sin and had imposed vicious punishments on it for centuries, left-handed people would have banded together as a quasi-ethnic group to demand freedom from such oppression, complete with "lefty" pride, parades, and indignant insistence that they were "born that way", to the outrage of religionists who would parade "ex-lefties" who had achieved right-handedness through faith in the Messiah. In that case our language would certainly have established a noun for a left-handed person, as I've been forced to re-purpose the word "lefty" in this paragraph in order to even describe what I'm talking about. In our actual society where left-handedness is merely a trait of no moral significance, as homosexuality was in the Classical world, the concept of a left-handed identity would seem baffling and pointless.
For that matter, human sexual desire comes in a whole kaleidoscope of variations, of which one's preference in the gender of one's partners is only one aspect. In our present society where everything becomes politicized and group identity is emphasized to an unhealthy degree, we're probably fortunate that most of these variations don't fit the group-identity model very well. Other battles, other strategies.
I'm personally very conscious of this issue because my atheism is the most definitive part of who I am, and I generally feel like I have more in common with an atheist in a foreign country than with a highly-religious American like Pat Robertson, and yet I'm also very aware of how absurd the whole concept is. Why is there even a word for "atheist", for a person who doesn't share a specific set of delusions? We don't have a word for a person who doesn't believe in unicorns.
Only in the context of the immense power and repressive character of religious insanity does this concept make sense. If unicorn believers had dominated Western society for centuries, torturing and killing anyone they believed the unicorns disapproved of, and fighting huge bloody wars with each other over slight differences of opinion about what unicorns' horns are made of, then rejection of the belief in unicorns would surely have emerged as a central part of the struggle for a better world, and there would be a word for people who didn't believe in them.
But we mustn't confuse strategies and necessary defensive measures with the goal. It would be a terrible failure if, after the final overthrow of Christianity and conformist traditional values, we were left with a sort of Ottoman-style millet society which defined itself as a jigsaw puzzle of discrete communities (gay, black, Anglo, neo-pagan, etc.) with each individual categorized as merely a member of one or another such grouping. The only real success will be a society where individuals are free to be themselves, with whatever combination of traits, desires, and quirks make each person who he or she is, but not defined or categorized by them.