Not true.
Tbh with you I'm tired of hearing this opinion on this site, it's everywhere here.
Marriage is defined by one's society. Marriage in the middle east looks different than marriage here, marriage in India looks different then marriage in the middle east, marriage in OT times looks different then marriage now. It changes with time and culture, has different attributes (for example in India they have arranged marriages), etc.
Culture's define marriage, authorities in place define marriage as authority is instituted by God. The only current marriages I would consider invalid in our times are gay marriages, because they contradict God's idea of marriage. That does not make our marriage institutions invalid however, similar to how the apostles commanded us to obey all authorities except when they clearly contradict God.
If that weren't the case that marriage changes with culture, then Christians are committing adultery and fornication all the time - something we are commanded by Christ and told in many places in scripture not to do. Sexual immorality is condemned and is to be avoided at all cost, so if marriage isn't clearly defined it will always cause problems. We would not be commanded not to do something if we couldn't clearly know what it is we aren't to do.
We aren't our own authority. While wisdom is one of the highest things to be sought after, wisdom that contradicts scripture is not wisdom or knowledge at all. Perhaps the trains of thought that led one there, but there's also foolishness involved. The Bible, Christ, authorities that are in place, etc. have power. We are to submit to the authorities and powers when they don't contradict God.