Where did marriage come from? The Bible. It was created by God at the beginning of human history, in the second chapter of Genesis.
The laws defining and governing marriage are contained throughout its pages: restricting sex to marriage; forbidding divorce; placing the man as the head of and provider for the family; enjoining parents to love and teach their children; putting children under the authority of their parents.
People can vote on these things if they'd like, judges can ignore them, governments can define and redefine. But God's laws of marriage do not change.
When we break those laws, we suffer. To see the proof, just look around.
Our "traditional" marriages have trashed God's laws to the point where fornication, cohabitation, adultery, divorce, shattered homes and fatherlessness are epidemic.
Many people recognize the resulting crises and are desperate for answers: how to have harmonious marriages; how to control wild kids; how to bring sanity to their home lives.
Sadly, too few look to the creator of marriage and family for answers.
Advocates of same-sex unions are fully ignorant of how marriage even came to exist.
They totally ignore God's wisdom in creating humans to produce through male-female sex, which he safeguarded within marriage so every child would grow up with his father and mother, having strong male and female role models.
They have no idea of the profound, inspiring purpose for which God created marriage and family in the first place.
They can dress up what they're doing in the language of love, and say it will help "fix" marriage and family.
But this bullying, freight-train crusade to stretch the definition of marriage is a supreme example of a "solution" far worse than the problem.
Throw God out, this is what you get.