So when they say I do in front of witnesses and God it is not ligit?
If a man holds a gun to a woman's head and forces her to say "I do", are they married? No. Because there was no true consent on the woman's part.
That is an extreme example -- that forced marriage is not true marriage. This is also why child marriage is not true marriage, even if an 9 year old says "I do" to her 30 year old "husband",
it doesn't count. If you wake up with a hangover and a marriage certificate from an Elvis impersonator, it doesn't count.
But there are other ways in which the wedding can be defective.
For instance, what if one of the spouses was already married? They could not marry another person but rather the new marriage would be invalid because they were not really free to marry. This is often true with couples who have been civilly divorced.
If one of the spouses has no real intention of committing themselves for life but just wants a
"starter marriage", that's not really a marriage because marriage is for life. If one of the spouses doesn't want kids, that is not really a marriage because marriage exists for the procreation of children. A couple who gets married with no intention of monogamy (ie. polyamory, swinging, open marriage, etc.) doesn't really marry. A marriage contracted entirely to obtain a visa with no intention of actually living as a married couple would be invalid. A spouse who conceals something very important (say, that he is a felon on the run, has AIDS, cannot have children, is addicted to cocaine, is a homosexual, etc.) commits fraud and therefore they do not marry (this would also be grounds for a civil annulment).
These are cases in which the marriage was actually invalid, that nothing happened when they said "I do". Therefore, they were never actually married. There had to be some defect
at the time of the wedding which would render the marriage invalid in order for a (valid) Decree of Nullity to be granted. The fact that your husband was a jerk or cheated on you doesn't count. What God has joined together, let no man rend asunder -- there is no such thing as Christian divorce. No power on earth can break a true marriage (ratified and consummated), no matter what civil courts declare.