He can write and was a transgressive powerhouse long before Hogg. His mainstream novels are worth reading up to a point. Unlike current Hugo nominees, his work was genuinely speculative and broadly applicable while also going into touchy subjects and psychologies. Dhalgren is overrated unless you're into that whole genre.
He was very open about being molested as a child and then groomed as a teenager, and positive about the latter. Throw in the fetishization of raunch and you have long-standing gay issues and erotica all rolled into one. It's something few talk about directly and rarely is it discussed openly or shown outside of literature/porn that no one else reads. Probably seems obvious here and now, especially here, but it likely led to this discussion. Fags and squares are extremely uninformed on the topic and while the grooming is now a tale as old as time and one that even the community does something about when aware of it, the whole thing sparks very uncomfortable debate in every circle it comes up in.
Leftoids hate it because it's what they do, spelled out on a page in piss and cum. Everyone else hates it because it's fucking disgusting and, I believe, there is an element of culpability in letting it happen or not doing the right thing to prevent it or after the fact. At least that's my interpretation of various debates and the general response to Delaney. He made a few extremely provocative statements and left everyone else to fight over the solutions.