LGBT rights weren't on the radar of the average marxist

Plenty of German leftists, Marxist or otherwise signed a petition, in the 1890s, opposing Paragraph 175 of the German Legal code that criminalized homosexuality, including Albert Einstein, August Bebel, and Karl Kautsky.

Queer activists, like Karl Heinrich Ulrichs and Magnus Hirschfeld, actively sought out far left politicians in their attempt to repeal the law.

Bebel, who was the one to sponsor the bill to repeal paragraph 175, continued to be an advocate of women's and queer rights throughout his life and career.

Alexandra Kollontai was Bisexual and opposed the criminalization of homosexuality under Stalin's administration.

Harry Hay, who would found The Mattachine Society, one of the first gay rights groups in the US, was organizing farm workers for the Communist Party as far back as the 1930s.

Queer issues were definitely on the radar of plenty of Socialists in the early 20th century.

This argument gives the same vibes as "but everyone was racist back then!" arguments that American liberals give to hand wave away past injustices.

If we're to be thoughtful dialectical materialists about this: while queerness has always existed, and cultures throughout history have had queer subcultures, such as the Kathoey in Thailand or Molly Houses in England, the development of Capitalism brought with it a trend towards a more systematized, wider reaching regimentation of reproductive labor, then what had been seen under previous forms of class society.

On the one hand, this brought about the categorization and subsequent oppression of queer people. But on the other hand, industrialization brought people into urban areas, socialized labor, and allowed queer people to form larger communities, who could start organizing politically on a large scale.

Since the Soviet Union had not industrialized, that pressure on queer people in the Soviet Union, to organize at a large scale, didn't exist. And the prevalence of queer organizing in the more industrialized west, brought Stalin's administration to make the idealist error that queerness was an outgrowth of "bourgeois decadence", rather than material conditions.