ACLU does quite a bit more than just support gay marriage, though. If they do a lot of things you think are great, and a few you're meh about, it's still good to donate.
The people who donate primarily for the gay marriage fight don't get to earmark their money either.
I don't see why they don't make it possible to earmark money - I think they would end up getting a LOT more funding:
People that don't want to fund gay marriage advocacy would donate to the issues that they find more important, and people that feel strongly about gay marriage would donate more because they know it would all go towards their pet issue.
The people who donate primarily for the gay marriage fight don't get to earmark their money either.