Dating more than one person is often called playing the field. People do this because they are afraid they will miss out on finding the right person. Rather than concentrate on having one good relationship, they date as many people as possible. This often does not work out well for any of the people involved.
When playing the field is done, someone will eventually get hurt. This is often true whether or not they both people have an agreement to openly date others. If they don't have an agreement to date multiple people, one person will feel rejected when they find their partner is seeing someone else. This can lead to a dangerous situation if the rejected person feels they have been betrayed. Overall, it is best to concentrate on one relationship at a time. If it doesn't work out, then it is time to move on to dating someone else.