Would redefining marriage to mean any consenting relationship put children at risk?

Since prehistoric times and in cultures not influenced by Judaeo-Christianity, marriage has meant the mating union of a man and woman. Marriage has brought clear advantages to children who can count on both mother and father for material support, education, and nurturance through two decades of immaturity. Would a degraded definition that allows any consenting relationship to have the legal status of marriage endanger children's welfare?