It’s because, having been so “oppressed” for so long, the ultimate defiance of that “oppression” (getting married, which conveys state recognition upon the union) can also be used to drive their “oppressors” from the public square.
To many homosexuals (as is true with mant other “oppressed” groups) simple conveyance of equal rights isn’t enough. Those who oppose such rights, regardless of reasoning, must be punished for doing so. Both as a matter of revenge and as a warning/example to others.
These days even I don't need spiritual eyes to see the holy war raging around us, over and for us.
These motivated "oppressed" are the walking dead casualties of that war, just waiting for their physical bodies to die, too.
From what I've read, their revenge will not be served cold, but rather with eternal fire. Some victory that will be.