There are many anarchists and marxists who dismiss identity politics and long for the return of a historic left they’ve imagined where class came first and everything was so much better. These same people just so happen to be ignorant about the huge amounts of sexism, racism, and queerphobia that existed within the historic left.
To take just one example of left wing prejudice, in France from the 1930s to 1960s the famous libertarian socialist Daniel Guerin was forced to hide his homosexuality from homophobic socialists. As he himself put it,
There were within me two men and two lives. In one life, I was exclusively an activist and in the other I was, depending on the period, more or less tormented by my homosexuality, but there was never any link between my two selves. I certainly refrained from broaching the subject in front of any labour activists. . . . If other comrades were living with similar problems, it was only much later that I found out. There really was no interference between my two lives. (Quoted in Berry 2004, 17)
During this period several French socialist groups explicitly opposed homosexuality and were even violent towards homosexuals (the Maoists being the most violent). As David Berry puts it,
[Guerin] is emphatic about the abject misery caused for him personally and for all those in a similar position by the constant fear of being discovered and unmasked by a comrade whom one respected and admired, of losing their respect and even of becoming scorned and loathed. One was forced, at all costs, to remain silent, to dissemble, to lie if need be, in order to preserve a revolutionary respectability whose price could be measured only in terms of the abjection one risked falling into if one dropped the mask. (ibid, 18)
Identity politics is not ruining the left. The left has already been ruined for marginalized people by all those radicals who oppress those around them while claiming to struggle for freedom and equality for all. Identity politics is thus an essential corrective to historic and on-going flaws within the left. Flaws whose cost was and is the intense and long term suffering of already marginalized people within the very spaces that should have been fighting for their emancipation.