I'll assume that you are asking serious questions and want a serious conversation. No offense is intended, but I'm not interested in the sort of trolling conversation that often happens. It would be great to have an intelligent discussion about it!
The fundamental answer is the issue is not discrimination, but power; discrimination is just one bad outcome of the problem of political power. Minorities naturally lack political power (being outnumbered) and therefore are politically vulnerable and subject to abuse, including discrimination. There is a long history of terrible abuse happening that I don't have to recount, but a conceptually perfect example is some U.S. state legislatures where the majority votes to restrict voting by minority citizens. Protecting minorities so they have the same liberty and opportunity as everyone else is the objective.
It's philosophically and rhetorically interesting to call it all "discrimination" and debate the differences, but about theory of language - what meanings does a word have and to what extent does the using the same word mean that the things we describe are the same; the map is not the territory and that's still talking about the wrong word. The issue and the word, in practical reality, is power.
>> politics is dominated by heterosexuals
> For years we fought against discrimination based on sexual orientation because it wasn't relevant. Were we wrong? Is sexual orientation an important aspect of a person's identity that should factor into our decisions?
Again, it's a power issue. People who don't have a seat at the table of power generally don't have their concerns addressed; that's a fundamental reason why democracy is important - it's the only way to address everyone's issues. They aren't addressed because hateful people can abuse the minority, who have no recourse without power; because others don't care about the minority; and because even good-willed people don't really understand what the minority needs or has experienced. I learned the same in IT - the system will be designed to meet the needs of the people at the table; if you exclude department X, their needs will be mostly ignored.
Heterosexuals have plenty of seats at that table (almost all of them!); their needs and experiences will be addressed. The idea is to give others a voice too. One example that comes to mind is the AIDS crisis in the 1980s, where governments for years ignored an epidemic killing masses of people, but those were people without a seat at the table. Another is stop-and-frisk practices in some US cities, where African-Americans in poor neighborhoods are humiliated by law enforcement; if it happened to someone with access to power - if a banker on Wall Street was made to drop his pants for a search, the program would end before he or she zipped them back up.
The same issue, the power issue, applies to the other points in the parent, so I'll stop here to avoid redundancy.
> what harm does it do when people equate criticism with violence, and try to be "safe" from it?
I don't see this happening too much. Could you cite some examples? I usually see it applied to hate speech and other actual discrimination. If it is happening more broadly, what do its practitioners say? I don't know enough to understand it.