A voluntary, non-binding plebiscite isn't a "vote", it's an opinion poll. How did we get into this mess in the first place? The government just upped and changed the Marriage Act in 2004. No plebiscite, no "vote", no nothing other than politicians unilaterally making a decision to actively discriminate against part of society.
We don't have plebiscites for far bigger issues so why for this particular issue? It makes absolutely no sense.