First, this is better for the parents (which in turn is better for the children) because there is usually more support readily available. Second, from an attachment theory point of view it is better if children are raised in a polymatric family rather than a monomatric family.
When marriage occurs one marries the individual person and also marries their family structure all their attachments and relationships as well.
They are barriers people often use to avoid getting into relationships. There are many and varied reasons why people avoid forming an intimate relationships with a mate.