Really, it can't be debated? This is the US, and this is a discussion forum; OF COURSE it can be debated!
So, if you read the Constitution, you also find that the Supreme Court has the responsibility of deciding cases and controversies BETWEEN the states, which is EXACTLY one of the issues that had to be resolved. Another issue is that as a matter of civil rights, under the 14th Amendment (which despite some rhetoric to the contrary, IS part of the Constitution), the federal Supreme Court DOES have jurisdiction to rule that state as well as federal laws, and private practices of individuals and organizations, may violate the rights of individuals, and therefore must be ended or modified.
It WAS a state issue, until 1) there was a conflict because some states recognize such marriages and others refused to (another part of the Constitution spells out that the states must give full faith and credit to the laws and actions of the other states), which resulted in people being legally married in one state, moving to another and not having their marriage recognized; and 2) until citizens who were affected asserted that their civil rights (rights that are not specified in the Constitution, but are among the unenumerated rights retained by the people and the states under Amendments 9 and 10, and which individuals and groups can ask the state AND/OR FEDERAL government to act to protect) were being violated because states were not recognizing the laws of other states.