Are remarriages without an annulment mortal sins, and why have annulments exploded in number?

The only mercy you are going to get from God is if you repent of the remarriage and leave the spouse you remarried when you should not have. As long as your original spouse is still living, you are still married to them; that has been Catholic doctrine from the early fathers to the present, and you can read the nitty-gritty in my Matthew commentary. The church allows you to separate for very serious reasons, such as being beaten or an unfaithful spouse, but you cannot remarry while the first spouse lives. The Protestants allow remarriage after divorce for any reason because they read Matthew 19:9 differently, a very difficult verse, so this is an instance where you had better have the right interpretation. Before the 1960s the Catholic Church may have granted only about fifty annulments a year worldwide, because they took Jesus seriously, 'what God has joined together, let no man put asunder.' Then everything relaxed and now there are thousands per year. One thing that changed was the definition of marriage: the 1917 Code of Canon Law understood marriage primarily as the state in which you produce children, while the 1983 Code made procreation secondary and put the interpersonal relationship, love and affection, first. Once you base marriage on an interpersonal relationship, annulments become a matter of course, because anyone can say they are not getting out of it what they thought. That personalism came from John Paul II. The ontological purpose, the bearing of children, comes first; the interpersonal relationship is the effect.