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 summary, deep friendships for hundreds of years were embraced as powerful, robust expressions of oneness coexisting with another sacred union, marriage.”