At what point was it decided that "fornication" means specifically p.i.v. sex that occurs before formal vows are exchanged?
What Jesus said on the topic is that if a man has sex with a woman he isn't married to, he should marry her. This is was basic bit of standing up for women's rights at the time as a woman who had had sex, even if unwillingly, was doomed to be unmarriagable and to live in destitution. Jesus's commandment isn't ideal in some circumstances but it was a more liberal view than the prevailing culture.
This could be interpreted that in God's eyes, the start of the physical sexual relationship should be considered as tantamount to marriage, and that if you are having a physical sexual relationship then you should get married, but it's not specifically sinful for the marriage to take place after the commencement of the physical sexual relationship.
In this context, the meaning of Fornication could be deemed to be indulging in physical sexual relationships with no commitment, no expectations or even hopes of marriage. Someone whose sexual ethics are to date for a longish time without having sex, and who only starts having sex when they are reasonably convinced that this is "the real thing" is not committing that sin, even if they start living together while they save up for a great wedding day.
But even if that's wrong, the "sin" is attached to the breakup of the previous sexual relationship that didn't lead to marriage. If that was fornication, then that is something attached to that previous relationship. The current relationship, which is being celebrated in the church ceremony, is therefore by definition not fornication because the marriage ceremony is taking place.
Most churches expect couples getting married to do some kind of marriage preparation classes with the minister. This would usually include an opportunity to reflect on the hurt and sin and damage caused by each person's previous relationships before coming to this point, which would be individually and sensitively handled rather than having a one-size-fits-all teaching because some people will come to marriage from a history that includes being abused, and even Jesus's teaching as above should not be interpreted as being a command for a woman to marry an abuser. But where appropriate the minister will lead the couple through a process of repentance and healing, which onlookers will not be updated about.
Fortunately, God's love and forgiveness is a lot bigger than some bible-believers would like it to be. My biggest objection to your OP @onlytwo is the implied suggestion that sinners should be excluded from the sacraments of the church, and that if there was some kind of sexual immorality in the past of one or both members of a couple, that this creates for them an irredeemable status that means they should be excluded. That concept is way more heretically opposed to the teachings of Jesus than the nuances of the exact meaning of the word fornication.