In the OT marriage was:
1) having the consent and paying the girl's father,
2) having sex with the girl.
3) being exclusively her husband for life (otherwise the marriage is no longer valid)
If you wish, I replace one word: marriage was:
1) paying the father of the beloved young woman,
2) having sex with the chosen and beloved woman.
3) being exclusively her husband for life (otherwise the marriage is no longer valid)
Question: Where does it say ceremony is a must? A marriage is valid before God if two of the opposite sex love each other, want to spend their life together, have the consent of the parents, NO?
Answer
The Bible doesn't give all the details of what was involved in marriage in the Old Testament. As you say, we know that a sum of money may have been paid to the father, and the father paid a dowry to the bridegroom. It was probably not considered a marriage until the couple had sex. And the couple was to be exclusively each other's for life, although we do find multiple marriages as well. Two things that you left out were that the marriage was before witnesses. Deuteronomy 22:13-21 talks about the "tokens of virginity." Apparently there were witnesses who observed the couple go into the tent, and then took these "tokens" after they couple had sex for the first time. The other thing you implied, but didn't state specifically, was that the couple made a public declaration of their intent to be married. Among the Jews this took the form of a "ketubah," a marriage contract, somewhat like our marriage license.
I do have one slight quibble, also, with your third point. If either was not the other's exclusively for life the marriage could be invalidated by divorce, but was not automatically invalid. It was the choice of the parties to the marriage whether that would make it invalid or not. One party could cheat on the other and the marriage continue with the consent of both.
The Bible does not say that there must be any sort of ceremony. It doesn't give any details of what constitutes a marriage. Therefore it appears to leave it up to the laws of the land in which the couple lives. In the New Testament, Christians are told to be subject to the governing authorities (1 Peter 2:13-16; Romans 13:1-6). Thus, if the government requires a marriage ceremony then the Bible requires a ceremony. In the United States forty-seven of the fifty states require at least the signing of a marriage license before a person authorized by the state to marry the couple, and the signatures of witnesses. I am not sure of the laws of Canada. Mexico requires a ceremony. Many European countries require a separate civil ceremony even if a church wedding was performed.
From what little the Bible does say it appears that a marriage is valid before God if two of the opposite sex agree according to the laws of the land to spend their life together, usually with the consent of the parents. Such consent may not be required if the woman is no longer living with her parents. And love is not a requirement; many arranged marriages are valid even though neither party knew or loved each other before the wedding. Love as a prerequisite to marriage is a modern invention.