I don't think that they are getting demonized for wanting that (everybody wants that).
The problem is simply a lack of money. Giving the public sector workers their pensions requires money from the private sector. This sticks in the craw of private sector workers as money that could go towards their pensions or pay is instead going to be used on public sector pensions (which are way better).
It's a question of robbing the worse off to fund the better off. Absolutely appalling (and I would hope all progressives would agree).
Mustn't be a progressive. I have never been a public sector worker, only ever either been self-employed or working for a private company and I think (apart from the age thing and perhaps a cap) public sector pensions have been and should remain roughly as they are. As for them taking "private money" it's a two way street as the private sector can be said to be "taking" public money in the same way, for example the private sector don't have to provide health coverage to make them an attractive employer as that is paid for by public money.