That's a weird comparison. The solution would be to leave the ACLU. You can speak out against it, but you would have no legal or logical basis for preventing them from turning into the KKK if that's what the rest of the organization wanted.So if I join the ACLU, and all of a sudden there's a huge movement to turn the ACLU into the KKK, I shouldn't have a right to say "Listen, I joined the ACLU, I don't want it to become the KKK" because I'm not tangibly impacted?
I don't think you understand the 1st amendment. Basically, the first amendment allows people to peaceably: assemble with who they want, write what they want, say what they want, ask the government for redress, and practice their religion how they want. Allowing people to legally get married is not the same as requiring that other people believe gay marriage is "okay" anymore than allowing people to get married only for sex or money means that everybody else in society must think it is "okay" to get married only for sex or money. You would have a point if there were a legal basis to require churches to marry gay people, but there's not. Just like always, they can exclude people from their sacraments on whatever basis they want.Granted, I don't think there is merit to the position that marriage would be devalued as an institution if it were expanded to include gay couples. I also don't think there's any merit to supporting farm subsidies, or voting for Democrats, but since we have a first amendment, I don't get to make those decisions for anyone else. And I for damn sure don't support using the government to push people towards believing something they don't believe, which is the only reason one could possibly support gay marriage over the elimination of marriage as a governmental institution.
The relevant amendment to this is the 14th, which demands that the government treat people equally under the law, as marriage has become a legal contract.