Some people have given some good replies to the OP’s question, although it did seem a little confusing. The Church’s basic teachings regarding marriage are pretty simple.
–Sex is to be reserved for marriage.
–Marriage is for life.
These aren’t things that the Church came up with. These are the teachings of Christ handed down to us. At one time, not so long ago, every Christian denomination took them with absolute seriousness.
Since marriage is for life, there is no way that a valid, consummated marriage can be terminated except by death. The Church has no authority to nullify a marriage.
So an “annulment” is not something the Church can do. Rather it is a decision the Church can make, after an investigation, as to whether the marriage in question was validly made at its outset. Was there consent, freely given? Did the parties intend permanence, fidelity, openness to life? Was there some unknown impediment? If the marriage is found to be null from the beginning, the Church can declare that fact; but it can’t make the marriage null.
The teachings on fornication and adultery are also Christ’s teachings. Nothing the Church can do to change that.
Things just get complicated when sin enters in and human beings are involved. But that’s what reconciliation is for.