Bede the Venerable — as excerpted in the Catena Aurea on Mark 10:1-12

“(ubi sup.) In Matthew it is more fully expressed, Whosoever shall put away his wife, except it be for fornication. (Matt. 19:9.) The only carnal cause then is fornication; the only spiritual cause is the fear of God, that a man should put away his wife to enter into religiono, as we read that many have done. But there is no cause allowed by the law of God for marrying another, during the lifetime of her who is quitted.”