This is a series written by author Dan Brennan (you can buy his book here) on sacred unions and friendship.
“There was no question the biblical narrative speaks of a distinctive oneness in marriage where God calls Adam and Eve to enter a communion of “one flesh.” But it became obvious that physical or sexual oneness in marriage is not the only oneness or union the Bible speaks of. Marital or sexual union does not exhaust the richness of oneness in God’s story. Jesus’ prayer that we might be one just as he and the Father are one, suggests a rich, deep relational oneness.
In Christian spirituality, we could name marriages and these deep friendship stories, sacred unions. These friendships were not marriages. Yet, the witness of these stories is clear that friendship love can be as profound and passionate as any marital love. In fact, many of these friendships lasted much longer than many contemporary marriages.
In summary, deep friendships for hundreds of years were embraced as powerful, robust expressions of oneness coexisting with another sacred union, marriage.”