Does this last statement mean that if the couple fornicated before they married that their marriage can be annulled?
No. Sexual sin prior to a marriage does not automatically remove a couple’s ability to contract a valid marriage.
It means that marital relations that took place during the attempt at marriage are not retroactively declared to have been fornication and children born from them are not retroactively declared to have been born outside of wedlock. An attempt at marriage enjoys the presumption of validity until it is declared null, and sometimes even if a sacramental marriage was not present, a civil marriage was.
This is an important distinction to make because this unfortunate misconception can add greatly to the hurt that a family goes through when a marriage turns out to have been invalid. It is hard enough to find that a civil marriage did not have the qualities necessary to be a valid sacramental and indissoluble marriage. It is often a devastating thought, and for the sake of those who entertain it, every effort should be made to reassure them with the truth.
The family should at least know that the good faith present in the attempt did count in favor of the children and the good intentions that did exist in the couple–I do not mean that good faith is retroactively created in a one spouse whose lack of good faith was one of the reasons the marriage was not valid–and that it is not as if there was not even a civil marriage.
Of course, it is also possible for a civil marriage to be declared null, such that there really was nothing but good faith on the part of the innocent parties. If a man is already married such that even he is not rationalizing that his first marriage is not still in force, then he knows he is transgressing moral law by pretending marriage and having marital relations with a second wife. If his second wife is innocent of his existing marriage, she commits no moral transgression by having attempted a civil marriage with him. If both parties were to have known that one or both was not free to marry even under civil law, then of course they know their marriage is a total pretense and there really is no legal marriage and no good faith on anyone’s part. Whether civil law would literally say that when two people pretend a civil marriage when neither was legally free to marry that “a marriage never happened,” I do not know. I am sure that the distinction is very important when it comes time for a court to decide who owns the assets that the bigamous couple had accumulated, though!
Contrast a civil marriage, which the Church sees as an actual but possibly dissoluble marriage, with a marriage between two people of the same sex that is recognized by a secular government. The latter is not even a natural marriage. It is impossible that the Church will recognize it as a marriage. Now, the Church prohibits Catholics from attempting marriage when they are not free to marry by the laws of the state, provided that the state does not violate their natural rights by, for instance, prohibiting Catholics from legal marriage to each other. If a person were establishing their right to marry in the Catholic Church, all marriages recognized by the state would have to be dissolved in the eyes of the state, even marriage to a same-sex partner, but that doesn’t mean that the Church believes a marriage ever takes place between persons of the same sex. It simply means that Catholics are required to abide by secular law when they marry in the Church.
