If there's one thing I really won't tolerate, it's people either denying (or worse, defending) genocides and other atrocities committed by totalitarian regimes. I won't stand for right-wing crackpots saying that the Holocaust didn't happen, or "joking" that it was actually a good thing. And I won't stand for similar sentiments from the left either. So, just to make it absolutely clear what I believe:
Stalin was a mass-murdering tyrant and an absolute monster. He was responsible for literally millions of deaths, quite possibly tens of millions, and while there might be some dispute about the exact number, it doesn't matter. "He only killed 5 million people, not 25 million!" is not a remotely compelling argument. The Holodomor really happened, it was deliberate and not accidental, and the Ukrainians who were forcibly starved to death under Stalin's rule did not deserve their fate, nor were they somehow responsible for it.
All of this has been verified by countless researchers and historians from all over the world, and it can't be dismissed as "CIA propaganda" or "Western revisionist history" with any more credibility than the Holocaust can be dismissed as a Jewish lie. (This is true EVEN IF the CIA or Western revisionist historians DID exaggerate some of the exact figures; again, the death toll "only" being a few million and not tens of millions doesn't mean it was somehow acceptable.)Also, the USSR in general was a totalitarian nightmare, and while Stalin was by far the worst of its leaders, he was not the only problem with the Soviet Union or with Marxist-Leninist doctrine. Lenin himself was a brutal and oppressive dictator who espoused a repressive ideology based around violence and coercion, and his claims of fighting for a better future are at best irrelevant in the face of his actions, and at worst nothing more than self-serving propaganda. Trotsky was likewise an authoritarian warmonger, and the fact that he opposed Stalin (largely for not going far enough!) doesn't mean he's a figure we should idolize. If he'd actually succeeded at taking power, he would've been a murderous dictator too.
Mao was likewise horrible, and Communist China was (and, to a lesser extent, still is) another totalitarian nightmare. Same goes for Ho Chi Minh, same goes for Fidel Castro, same goes for Che Guevara: they were all violent authoritarians. Even the more "benign" Marxist-Leninist nation-states like Vietnam and Cuba - while not falling to the same depths of repression, horror, violence, and death as the Soviet Union, Maoist China, North Korea, and Cambodia under the Khmer Rouge - were still pretty awful. And while Vietnam and especially Cuba have gotten a LOT better over time, that's a direct result of the fact that they've moved away from Marxist-Leninist dogma and embraced liberal reforms.
In addition to the general authoritarianism of Marxist-Leninist dictatorships, all of the regimes I listed above were terribly homophobic and transphobic. The idea that ANY of them championed LGBT rights or were pro-LGBT in any way, shape, or form is not only 100% factually inaccurate, it's a sick and twisted joke. For instance, Modern Russia is still a terrible place for LGBT people (the majority of the populace opposes homosexuality, and the nation has laws prohibiting people from publicly speaking about LGBT issues), but the situation there is still leagues and bounds better than it was during the Soviet era, when simply being LGBT was illegal and could get you sent off to a forced labor camp for the rest of your life. China, both during the Maoist era and in the modern day, has been similarly hostile towards LGBT rights. Even Castro - who was far more tolerant than Stalin or Mao - sent openly gay and bisexual men to work camps, refused to let them join the military, and repeatedly stressed that being queer was incompatible with being a communist.
And yes, as tankies love to point out, a lot of people in the Western world are homophobic and transphobic too. And up until very recently, many Western countries have had laws denying equal rights to LGBT people. But 1. bringing that up in response to Soviet or Chinese oppression is just Whataboutism, it doesn't make what they did remotely okay, and 2. Western liberal democracies are VASTLY better in this regard, even if there's still plenty of room for improvement.