Follow up question from my friend over at the Catholic forum and my response. It was a very good follow up question!
Question: I'm still unsure about one thing. Say a non practising Catholic couple are married by a non Catholic celebrant and they die without ever having intended on getting their marriage convalidated, would this have any effect on where their soul ends up?
Answer: To my knowledge a Catholic couple (practising or not) being in an invalid/non-sacramental marriage would equate to committing adultery and thus being in mortal sin which would mean Hell if they both died with mortal sin on their souls (i.e. unforgiven and unrepented), but there's an important caveat to highlight. The precise nature of the sin will depend on what their will was at the time of the marriage, when they realised it was invalid, and whether they are having sex, etc., but the matter involved in this area is grave and thus, if done knowingly and deliberately, it will constitute mortal sin.
[Blogger's note: emphasis added; I forwarded a paraphrased version of the original question to Jimmy Akin; the highlighted portion is his response]
The caveat: Vincible versus Invincible Ignorance
If a Catholic couple was genuinely ignorant of the fact or unaware that they had to be married by the Church in order for their marriage to be valid and sacramental, then they may not be culpable (responsible) for the resulting sin.
"Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors." - CCC, 1735
"Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin." - CCC, 1859
"Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest." - CCC, 1860
So if the couple is genuinely (i.e. invincibly) ignorant, then they may not suffer the fate that which mortal sin presents, but if the couple indeed knows that being married invalidly would result in mortal sin and gets married invalidly in a manner of "rebellion" so to speak, then they will have willingly committed mortal sin and for that to diminish they would need that sin forgiven by the Sacrament of Reconciliation and for their marriage to be convalidated.