There was never a need to amend the Oklahoma Constitution to ban gay marriage. It's already in there in Section I-2.
Along with poligamy, plural marriage is forbidden. Plural, as you know, means more than one of the same kind.
This was done in 1909, has not been challenged, and still stands.
Woody
That's a pretty broad definition of the world plural there; I'm not sure monogamous same sex marriage was prohibited within that statement. It's never been challenged in you context, because your context is entirely irrelevant to the meaning of the word plural.