Harpers has a list, compiled by Perry Moore, of gay superheros and their fates. In short, it isn't pretty. This entry on DC Comics' Obsidian is fairly representative:
OBSIDIAN: depowered, corrupted by his sexual strife, manipulated by dark forces, thwarted in an attempt to destroy the world, made a security guard for a team of heterosexual superheroes but not allowed to sit with them at the table
In short, it's a long list of torture, rapes, shunning, abuse, and secret villainy not infrequently at the hands of people closest to the gay superhero in question. Whether it's out of sadism, a scapegoat mechanism, or the inability to write (or for the audience to accept) any other kind of storyline is open to debate.