If it's not a constitutional issue, that's exactly what they should do - evade and avoid it because it's a states' issue. The only time SCOTUS should grant certiorari on a non-Constitutional issue is to overturn a lower federal court ruling and remand to the state court.
Gay marriage is not a Constitutional issue, it's a states' issue.
Actually, it's a matter of the laws of nature and of nature's God, which precedes and supersedes both.
It’s a God issue and everyone knows it.
God created them male and female. “Go forth and multiply.”
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.