nethier the civil rights act nor the legalization of same sex marriage is as concrete as a constitutional amendment, which is itself part of the constitution, and determines whether other laws are constitutional.
the civil rights act was not the work of the democrats or the republicans. it was the work of dedicated activists and, yes, other parties such as the black panther party. they exerted pressure onto the parties in government, and the parties in government acquiesced.
Yes, it was the Democrats who both supported and are primarily responsible for its passage.
... in congress. but they would have never proposed it if the writing weren't on the wall, if the people in the streets had not made it a matter of import. giving the democrats credit is like giving bank tellers credit for financing bank robbers' lifestyles.
I'm all for people demanding things in the streets. That's not the same as what you've been suggesting, which is to vote third-party. These two things are two entirely different things.
Black Panthers didn't hold legitimate Congressional power like Democrats. So again, thank Democrats in Congress for passing it. Something you risk spoiling by letting Republicans get into office.