I was a member of the SDA when I worked in retail in high school. I was signed up, like this article describes, as a standard part of the onboarding process. When every new employee is signed up like this, the SDA ends up with huge membership numbers and consequently huge power to influence ALP policy.
At the time I knew that the unions formed a large part of Labor's support, and that this strongly influenced Labor's industrial policies. I also knew that there was a strong historical Catholic influence in the ALP. I had no idea, however, how much the SDA was meddling in social issues outside of industrial relations. De Bruyn's claim that his members agree "absolutely" with his stance against marriage equality is glib bullshit. They have no idea what he's up to and he knows it.
The SDA has no place dictating social policy. If I were still a member of the SDA today I would cancel my membership immediately. It would be delicious irony to see large numbers of current SDA members do likewise -- go on strike from the union, as it were.