I had that experience when I was young, dumb, and rebelling against the Establishment in the 1970s. I was busy with work and didn't follow politics, so I thought the ACLU was a nice bunch of folks helping our black neighbors get better treatment.
Then I got a fund-raising letter in the mail from ACLU listing all the things they were against and wanting to take down: the First and Second Amendments, the Boy Scouts, the laws around traditional marriage and parenthood, etc.
That card was the absolute turning point. I abandoned liberalism on the spot.
I joined the ACLU myself long ago thinking they were Libertarians.
Found out pretty quickly that they were not.