There's quite some space between opposing something and not finding something important enough to fund activism in favour of it.
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.
Or are you straight and hence it doesn't affect you? Kinda defeats the point of donating to charities...
I would like to not pay more because of whom I choose to marry.