So if divorce is possible, then what is divorce, according to the Catholic Church, and how does it differ from separation?
There are very limited cases where a valid marriage can be dissolved, but I suspect that as used, the term “divorce” is used to refer to the governmental dissolving of the legal contract and fair distribution of property, etc, and had nothing to do with the status of the actual marriage itself. Separation may or may not involve this process - a couple could be married in the eyes of the law and of God and be living in different houses.
As far as unintended fornication goes, I don’t see how that answers my question. Take for example:
Paul and Susan go get married. Now, Susan isn’t actually intending on remaining faithful during her marriage to Paul. After a few months, Paul discovers that Susan is cheating. He then must make a decision as to whether or not he will seek an annulment. Let’s say that in Possible World A, he does and Possible World B, he doesn’t.
A) It is discovered that Susan had no intention of remaining faithful and so Paul and Susan were never married. Annulment granted.
B) No annulment is sought out, and Paul and Susan remain as they were. Paul and Susan have sex, but as they are not really married (Susan violated one of the requirements for marriage when she did not intend to remain faithful), Paul and Susan are actually fornicating.
Since annulment doesn’t actually change whether or not Paul and Susan were married, why is my conclusion about fornication in B wrong?
To summarize: you want to know, in the case that there are grounds for an annulment and hence there is no marriage, A) if the couple would be fornicating, and B) if, should one of the spouses discover these grounds, they are then obligated to go through the annulment process.
I did a little research and didn’t find much (most of what I found was addressing more common situations), but there are a couple principles worth pointing out. First, sin requires knowledge. Second, marriages are presumptively valid. In most actual cases, I suspect these would suffice. In general, there is no need to question the validity of a marriage unless one of the parties seeks a different marriage, and until a decision is reached otherwise, the marriage is considered valid. Thus there would be no sin in relations, because as far as the spouses know (and as far as they are expected to know) they are married. This addresses A) in the absence of a sufficiently strong B) (ie mere hint of a suspicion that there might be something doesn’t necessarily obligate a full blown investigation).
But if one spouse gains certain (enough) knowledge that an impediment (of the sort you mention, not a past marriage or something) existed and yet does not desire already to separate, but would rather sustain the relationship and turn it into a valid marriage, need they do anything?
It’s an interesting question, and despite vaguely remembering it coming up before, I can neither remember nor find the answer. My actual advice (barring a better answer from someone else), should such a situation come up, would be to go talk to a knowledgeable priest.
