Why is that people feel the need to change the terminology to define a group of people?

For example, when I was a kid it was inappropriate to refer to someone as “queer”, I was told homosexual was more polite. Then it had to be gays and lesbians. Now I see people referring to THEMSELVES as queers. Same for race, when I was younger, nobody used the N-word, but “colored people” was okay. Then that got people mad, so it became African-American. Now that seems to be un-PC so I’ve heard of some black people refer to themselves as colored, or “persons of color”. Well, as a woman (who happens to be white), I will no longer tolerate; woman, female or ‘the weaker sex’. I wish to henceforth be referred to as a “vaginally enhanced human in a neutral shade of opacity”.
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So tell me, what gives? Why do we find the necessity to constantly ‘rebrand’ a group of people? I can understand racial and sexual intolerance, and certainly do not condone it, but it can be confusing for someone who is trying to be sensitive to another person’s identity.