Neal and Kevin,
Here are the posts I wrote on it in chronological order; I personally think #'s 1 and 4 are most helpful, but feel free to read all or none:
The Bible verses in question are Matthew 5:31-32 and Matthew 19:1-12 (no divorce except for cause of porneia); and Mark 10:11 and Luke 16:18 (no divorce, period). The word for adultery, moicheia, is used in all of those verses to say that anyone who divorces and remarries commits adultery. In Matthew's version, where a specific nook is carved out, he doesn't record the word for adultery ("except for cause of porneia"). In fact, Jesus lists porneia and moicheia as distinct sins in Matthew 15:19 and Mark 7:21. Paul lists them separately as well, in Galatians 5:19.
I mention this because most of the Protestant interpretations of Matthew 5:31-32 and Matthew 19:9 assume porneia to mean "sexual unfaithfulness to your spouse," as something either identical with, or broader than, adultery. This interpretation is unsustainable: they would be listed as a single sin if this interpretation were correct (even if one term is broader, it would include the other as a subset). Besides that, this interpretation flies in the face of Mark 10:11 and Luke 16:18, which prohibit divorce without exception.
In fact, Matthew's Gospel shows that divorce and remarriage is strictly prohibited unless you were never married in the eyes of God. To get there, we need the help of Blue Letter Bible's Lexicon/Concordance, LexiCon, which lists every usage of the word porneia in the New Testament. In addition to the uses which I've just mentioned, here are the other uses we see:
- In John 8:41, it means "fornication."
- In Romans 1:29, it probably refers to homosexuality (given the context).
- In 1 Corinthians 5:1, it refers to incest with one's mother-in-law.
- In Acts 15:20, 15:29, and 21:25, it refers to incest. The question is which parts of the Levitical law (Lev. 17-18) still need to be maintained by the Gentiles, and the answer is: Idol Sacrifices Lev. 17:8-9; Blood Lev. 17:10-12; Things Strangled Lev. 17:13-14; and Porneia (Incest) Lev. 18:6-18.
- In 1 Corinthians 6:13, 6:18, and 7:2, it refers to fornication (in that case, with prostitutes). 2 Corinthians 12:21, Paul says that they still aren't repentant of their porneia, which we can contextually assume refers to the same fornication mentioned above.
- Colossians 3:5 and 1 Thessalonians 4:3 list it as a sin of the flesh, but don't give much more detail. 1 Thessalonians 4:3-5 almost seems to equate it to "passionate lust like the heathen," but Colossians 3:5 lists them as separate sins.
- In Revelation 2:21, 9:21, 14:8, 17:2, 17:4, 18:3, and 19:2, it refers to Babylon's fornication/prostitution.
What this means in the marital context: there will be times where you can civilly marry immorally, whether that means marrying a relative, a member of the same sex, or a divorced person. Those marriages aren't valid in the eyes of God. Jesus refers to the first two classes as fornication, and the last as adultery. To put it in a modern context, if two men marry in a state which permits gay marriage, one of them converts, and wants to know if he can get a divorce from his (non-cheating) civil spouse, the answer is absolutely yes. That's so obvious that Mark and Luke don't bother including this detail, but Matthew (fortunately) does.
This interpretation harmonizes Matthew, Mark, and Luke perfectly. The operative phrase is found in both Matthew 19:6 and Mark 10:9: "What therefore God hath joined together, let not man put asunder." God never joins together siblings, gay couples, or divorcees in marriage. But those marriages which He does join can't be broken for any reason, including adultery.
So the question then becomes, "Was this marriage one which God joined together?" And that's a good role for the Church to play. She's more objective than the parties involved, and can make a final determination upon the status. We already know from Matthew 18:17-18 and 1 Corinthians 6:1-2 that the Church plays a judicial role in the lives of disputing Christians. Annulments then, are simply declarations by the Church that the civil marriage which had existed was, for some reason, null -- which is why they're called Declarations of Nullity. To the best of the Church's knowledge, there never was a marriage in the eyes of God in this instance, and the couple was, instead of consummating a sacred bond, simply fornicating.
P.S. I'll acknowledge up front that some of the people you present this to won't like how strict this rule is. The disciples' reaction (in Matthew 19:10 was, "If this is the situation between a husband and wife, it is better not to marry." So the fact that it's an unpleasant truth only further suggests that this is precisely the manner in which it's supposed to be read.
P.P.S. Turns out, Kevin's journal has an article on the evolution of divorce in the US: http://nationalaffairs.com/publications/detail/the-evolution-of-divorce.