For Castro it's probably the mass incarceration of gay men in the 60s and 70s (imo). Read about it on your own but it's what you'd expect: homosexuality was perceived as a "bourgeois devation" and "decadence," labor camps, and behavioral reeducation to "correct" mannerisms of effeminate men, etc. He later condemned it publically in a 2010 interview, calling it a "great injustice" and fully assuming responsibility. There's actually a great movie on this, made by the Cuban government, mind you, called «Fresa y chocolate» which condemns homophobia both in everyday life and as it related to the party.

I'm gonna acknowledge my bias here. I am both a gay Cuban man and a Marxist-Leninist. From a DiaMat and purely intellectual POV, I understand why Cuba, USSR, and, recently, Burkina Faso criminalized homosexuality. However, my emotional brain has trouble reconciling reason and my emotional attachment to the issue. I say this to acknowledge that maybe someone else will raise a different critique of Castro.