there's an option on the donation page where you can opt-in to the ACLU sharing your details with other charitable organizations. (the default is opt-out).
I verified via the wayback machine and the page is the same today as it was when I made my donation. I'm fairly certain I would not have checked that box. I make it a point to read checkboxes very carefully as companies are so prone to anti-patterns to get you to agree to share your info or join mailing lists.
Except, you know, they're a bunch of lawyers so any abuse of their own privacy policy is not really going to fly with them, especially from an optics perspective. It's far more likely that you did, in fact, click the checkbox, and just forgot about it than the ACLU deciding to ignore your request and forward your information anyway.