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So, who does determine the validity of a marriage?”
Simple, it has to be free and open and a contract entered into by mature, consenting people with no outside pressures or motives.
God must be involved.
The couple often knows that God did NOT join them. That is a matter of conscience and the formal Tribunals even place great weight on personal conscience decisions about validity.
Who witnesses said contract? And who can void said contract?
What you're stating reminds me of free love at Woodstock.
By the way, the "maturity" cop-out has now become a very popular reason many annulments are granted, even if the couples were in their 30's when they got married.