Monogamy is not the problem. The problem is moving too fast without a solid commitment. True intimacy warrants monogamy. Yes, that connection is so strong that two become so intertwined that a sense of ownership prevails.
The casual dating thing doesn’t work for everyone. No one is immune to jealousy and jealousy can be lethal.
If people want to suppress their feelings and adopt the alternative relationships then more power to them but I can’t believe it’s a happier state than monogamy.