DENVER—More than two years ago, Colorado was at the leading edge of the societal shift on same-sex marriage. Now, Colorado is on the sidelines as the neighboring states of Oklahoma and Utah await legal decisions from a federal court in Denver.

The careful, incremental approach to the issue in places like Colorado paved the groundwork for the dramatic changes elsewhere. Indeed, Colorado gay-rights supporters are not even committing yet to putting a measure on the 2016 ballot to legalize same-sex marriage.

The gay rights debate in Colorado dates back to 1992, when voters passed a measure barring any city or town from passing laws protecting gay rights. Colorado banned same-sex marriage in its state constitution in 2006.