That's not how it works, at least not in my mind. When dealing with something that is assailed as frequently as gun rights, lack of support is the same as opposition.
I want an organization that supports the Constitution, completely and fully.
But then I hope you're not sending a single cent of your money to NRA, either? I mean, they also don't "support the Constitution, completely and fully", right?
Or is that okay because they're explicitly scoped to the Second Amendment?
If so, then ACLU is in the same exact boat, being explicitly scoped to everything other than the Second. And then some: ACLU is the American Civil Liberties Union, not American Constitutional Liberties Union - US Constitution to them is a tool to aid in protecting the civil rights they care about, it's not a definitive list of rights that are worth defending.
ACLU position: Given the reference to "a well regulated Militia" and "the security of a free State," the ACLU has long taken the position that the Second Amendment protects a collective right rather than an individual right.
They protect civil rights, not whatever people who wrote the Constitution 200 years go thought civil rights should be.
It's my position that that ACLU is wrong and will never see a penny from me.