Dr. Ed Peters makes that claim in a recent post. I doubt it.
Any Christians who attempt to contract marriage without the intention to do as the Church does, including to commit themselves to a lifelong union before God, do not have a valid Sacrament. Any Catholics who attempt to contract marriage without proper canonical form do not have a valid Sacrament.
Any unbaptized persons do not have a valid natural marriage, if they do not consent to a lifelong union, or if they are not open to any of the true ends of marriage, especially procreation and the fidelity of the spouses.
Some authors opine that contracepted sex does not validly consummate a marriage. For the primary end of marriage is the procreation and education of offspring, which is contradicted by contracepted sexual intercourse.
In addition, any Christians who are divorced and remarried do not have a valid second (or third or fourth…) marriage.
We don’t know what the percentages are, but in this very sinful world, in which most persons are not Christian and most Christians are not faithful to biblical or Church teaching, I am very skeptical that most civil marriages are valid.
Please take a look at this list of my books and booklets, and see if any topic interests you.