An open relationship is a modern type of relationship or marriage; It is one in which both partners agree that one of them may have sex with other people.
Basic Definition: an Open relationship is similar to EATING your cake and still HAVING it.
Simply doesn't work, by the way there's nothing like an OPEN MARRIAGE, marriage has been reduced to just about anything including this nonsense idea of open marriage.
The plausible reasons people go into open relationship is
selfishness(they want to explore with others while having one as the default tire),
a deep rooted fear of commitment(when committed to something we tend to invest our time, emotions, finances and Whatnots, the fear is what if the relationship fails?), and
3)hyper sexuality(everywhere you look you see sex; from books to movies to TV series, even advertising isn't left out, we are being programmed to be promiscuous).
Threats in an Open Relationship
Higher risk of STD, and STI.
Lack of Commitment, and/or Loyalty.
Eventually nature, and feelings set in Leading to jealousy and bad blood.
People is open relationship tend to fail in a homogeneous marriage. Why settle for one when you can get 2 or more?
To conclude, just because it is modern doesn't make it good, the ONLY benefit is sex(which you can get easily without being in a relationship), therefore it makes no sense to be invested in an open relationship.