Why is Newt Gingrich ever asked his opinion on marriage?

Newt has been married three times, so clearly he is an expert.

If they're worried about threats to the institution of marriage, they should ban divorce.

Apparently not. Remember that the religious organizations that are opposed to same sex marriage also oppose divorce (fault or no fault) and remarriage of divorcees. No state or federal law has ever required that a religious organization marry anyone. If you don't believe me, ring your local Catholic parish and tell them that you are a divorced Episcopalian and you want to marry your Jewish fiancee in their Church and see what kind of reception you get. When I was more closely associated with the Catholic Church, most parishes would only book in weddings if one of the parties was a parishioner that regularly attended Sunday Mass.