At least some of them are afflicted with chronological snobbery. This is the argument that the thinking and by extension, the attitudes and cultural mores of earlier times are inferior to that of the present. This can be seen in statements like, "It's 2017. Get with the times," and the description of same sex marriage opponents as "dinosaurs," or "on the wrong side of history." On social media, reacting to a picture of a group of mature aged people attending a "No" campaign event in Sydney, one particularly nasty person slurred them, saying in as many words, that they should roll over and get out of the way, so that younger and more "progressive" people could run things.
To this we can add latent racism. One letter writer noted the large "no" vote in Australian electorates with high migrant populations. According to the writer, their vote was influenced by the cultural attitudes and religious beliefs of their countries of origin. In other words, if they were better educated and more Westernised, they would have voted differently. The latent racism comes from what appears to be the attitude that Western values are always superior to those of other parts of the world.
Leftists often like to make out that they are superior to and more virtuous than conservatives. Watch them closely enough, and you can see through their facade. In their own way, they are just as much the bigots they accuse their opponents of being.