The relevant Gospel passages have always guided the Church's teachings.
Last year, Pope Francis called for an extraordinary general assembly to address “the pastoral challenges to the family in the context of evangelization.” It hasn't even begun and there are already rumors that this might lead to the Catholic Church overturning Her two thousand year old prohibition against divorce and remarriage, at least as that prohibition applies to cases involving adultery.
For those who think that the Church's teaching can and should change, the strongest argument comes from Matthew 5:32 and 19:9, in which Jesus seems to carve out an exception in His teaching against divorce and remarriage:
– Matthew 19:9. The RSV:CE reads: “And I say to you: whoever divorces his wife, except for unchastity [πορνεία, porneia], and marries another, commits adultery; and he who marries a divorced woman, commits adultery [μοιχάω, moichaō].”
The Protestant website GotQuestions? is hardly alone in claiming that these verses “allow divorce only in the case of the adultery of the other party.” Indeed, virtually every Protestant denomination permits divorce and remarriage in at least some cases (as does the Orthodox Church, sort of). So does Jesus permit such exceptions in these two passages? And can we expect the Church to change Her teachings to allow such an exception?
The answer to both questions is a resounding no. Here's why:
That is, the “unchastity” in question isn't Bob cheating on his wife Alice. It's the unchaste relationship between Bob and Alice themselves. If Alice and Bob are civilly married, but aren't married in the eyes of God (for example, if they're siblings, or of the same sex, etc.), they are free to “divorce” and remarry.
Now, I realize that this interpretation flies in the face of what you may have heard, but the case for it is iron-clad. Consider the following evidence:
Relatively few Biblical doctrines turn on the precise Greek wording of a Biblical passage. This happens to be one of them. Fortunately, it's straightforward.
Jesus talks about divorce and remarriage being adultery except in cases of “porneia.” Protestants tend to claim that porneia means an exception for “adultery,” but that interpretation can't be right. There is a Greek word for adultery, and this isn't it. Jesus actually uses the Greek word for adultery, moichaō, in both Matthew 5:32 and 19:9, but only to say that the divorced-and-remarried person is guilty of it.
In fact, the word porneia is never translated as adultery in the New Testament (at least in any of the major English versions of the Bible), and it's used to describe a variety of sexual sins, but never simple adultery. Jesus also explicitly distinguished porneia from adultery (condemning them both, but as separate sins) in Matt. 15:19 and Mark 7:21. Paul also distinguishes porneia from adultery (Galatians 5:19).
So, having just seen that porneia never means simple adultery, we're left asking: what exception is Jesus allegedly creating here? GotQuestions claims He is allowing “divorce only in the case of the adultery of the other party.” But where's the reference to anything about it needing to be the other party's adultery? GotQuestions is adding something to the text that isn't even remotely there. Furthermore, how would that work, exactly? Bob cheats on Alice, so she divorces him and remarries, and he's left in some sort of weird marriage limbo, where he's no longer married to Alice, but isn't allowed to be married to anyone else, either?