Can anyone explain the gay HIV epidemic of the 80s?

The AIDS crisis is a pretty complicated segment of history, cutting across a lot of different areas - medical/biological, public health, social activism, LGBTQ liberation, as well as race, gender, class, etc.
I'm going to give you a pretty quick run down of the main points as well as a more qualitative analysis of the situational and emotional forces at work. Feel free to ask any follow-up questions.

The first reported cases of what we now know to be AIDS (Acquired Immune Deficiency Syndrome) occurred in clusters of gay men in New York, San Francisco, and Los Angeles in June of 1981. We now know there were cases before then, and that HIV was already present in the "collective gay bloodstream" (to use a metaphor) in the late 1970's. The challenge of identifying AIDS as a unique illness is that it is a syndrome caused by the collapse of the immune system. As such AIDS can look like a bunch of different things. There are dozens of illnesses that are common in people with compromised immune systems; several of these came to be known as 'AIDS-defining illnesses' - pneumocystis pneumonia, Kaposi's Sarcoma (KS,) Toxoplasmosis, Cytomegalovirus (CMV), and others. Because of the variety of ways in which AIDS could present it took some time for doctors and researchers to identify the cause of AIDS - the virus HIV.

No small part of the slow progress was the fact that AIDS was affecting primarily gay men and injection drug users (as well as a small population of Haitian immigrants and hemophiliacs affected by infected blood products.) Although gay men had gained a very small amount of social tolerance during the decade preceding the AIDS crisis, there was still a substantial amount of inherent social stigma towards homosexuality. This meant that relatively few medical professionals and researchers wanted to have their names attached to working on the 'gay disease' (however there were very many who did, often to the initial detriment of their careers.) More importantly there was extremely little funding, particularly on a national level, during the early years of the epidemic. It was the early years of the Reagan administration, and a time of fiscal and social conservatism.
These factors meant that the AIDS crisis was an incredibly dire time in American history. Gay men felt abandoned by their country, and to a large extent they were. It was by no means uncommon for gay men with AIDS to lose their jobs or be evicted from their apartments (or be turned out of their lover's apartment after his death.) Homophobic violence or "gay bashing" increased. There was substantial misinformation about how HIV was spread, causing a lot of AIDS panic and hysteria. Even without all this, the sheer numbers were staggering. It was not uncommon for gay men to have dozens within their social circle - lovers, friends, friends-of-friends, colleagues, acquaintances - die. Writer, playwright, and activist Larry Kramer wrote in 1989 that he estimated his number dead acquaintances at 500. Obviously that number is pretty staggering, and likely not indicative of every gay man, but it I include it just to show how truly massive the death and dying was.