Marriage becomes a federal issue if citizens of one state try to compel another state to recognize their relationship. Further, while it is often bad for courts to be overly proactive in addressing issues which are not relevant to immediate cases before them, accepting petitions for declaratory judgments is often good, since it allows people to know how the court would be likely to interpret various actions they might consider pursuing.
That doesn't make the subject matter a federal issue. It only means upholding the full faith and credit of the state from which its citizen(s) journey to another state.