Most people are too dishonest with themselves and others to be with multiple partners. Most people when they have multiple partners are dishonest about it.
For most people, it is best for them to be in monogamous relationships versus being with multiple people.
When you are in a monogamous relationship, there is a level of integrity that is called upon. You get to know the other person better. Trust and honesty are built up. You can share with them the most intimate parts of you. Being in a monogamous relationship offers support for each person.