Same-sex marriage is a federal issue. Just one example: Opposite-sex couples don’t have to worry if their marriages are recognized as they travel across the country or move from state to state. Same-sex couples do.
Here’s same-sex marriage advocate Mary Bonauto reacting to the disappointing election results in Maine in Time:
Ultimately this is going to have to have a national resolution. It’s about aligning promises found in the Constitution with America’s laws.
… nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.~ United States Constitution, Fourteenth Amendment, Section 1