My position that fornication is grounds for divorce is based on the idea that the word porneia in Matt 19:9 should be translated as fornication and not as adultery. Although it can’t be proven, I agree with the supposition that Jesus would have been more likely to use the word moicheia if he meant simple adultery.

John Piper's position paper on divorce and remarriage is a decent introduction to the idea. I don’t agree with everything in Piper’s paper, but he introduces this idea acceptably well.

Thayer’s and Smith’s Bible Dictionary defines porneia as:

illicit sexual intercourse adultery, fornication, homosexuality, lesbianism, intercourse with animals etc. sexual intercourse with close relatives; Lev. 18 sexual intercourse with a divorced man or woman; …

As you probably know, porneia is translated as sexual immorality in some modern translations (e.g. the ESV). IMO, the problem with translating porneia as sexual immorality is that sexual immorality is too broad and general and doesn’t adequately convey the meaning of porneia. Even though fornication is not an exact translation for porneia, I think it is better than adultery or sexual immorality.

- Al