In partriarchal times marriage within the can was the norm. Marriage within certain degrees of relationship was forbidden, but monogamy only gradually became the rule, and divorce was tolerated before Jesus' prohibition.

a completely unique relationship that we humans experience

a relationship, and a relationship consists of mutual expectations

a relationship between equals

a relationship expected to be marked by honesty and openness

a sort of agreement, unless other options were agreed upon by the couple, to be with eachother, exclusively, until death or divorce (or anullment)