Equal treatment under the law is the civil right being discussed here.
Marriage? I don't care if they call it "bombaloo". The civil union under the law is what needs to have equal application. Currently, it doesn't.
If any state decides to do that, what do you care? If there is a church somewhere that has performed same-sex marriage, call it "marriage", and intends to continue, you are perfectly free to avoid that church.
Are you really interested in amending the Constitution to include a vocabulary exercise? When was the last time the Constitution was amended in response to a social issue of the day? Hint: it begins with p-r-o-h-i-b-i-...
And what a success that was.