Is it just me, or has the ACLU been on a roll lately? I'm really enjoying its current direction. It might have just been that the most controversial headlines were getting the most airtime, but it seemed to be going off the tracks for a while.
I personally don't hold the constitution as a sacred document, though I do find that it aligns with my ideals more than the actual policies of modern America. I could respect an organization being opposed to an amendment and deciding not to defend it for that reason, but it's always been weird to me that they have to hide behind a legal rationale rather than just admitting their political opposition to individual gun rights.
I still love the ACLU, and in the past I've given them time (passing out flyers, IANAL) and money despite these reservations. I'm really not trying to hate on them, but I would consider them "off the tracks" in this regard.
You're leaving out the third option, practical.
There are already well funded, effective organizations defending your interpretation of the Second Amendment. It makes little sense for the ACLU to divert resources to that endeavour as a result, even if they agreed with your political view on the individual vs. collective right to bear arms.