It's all personal preference. If 2 people involved in the relationship are happy for the other to see someone else/sleep with others, then there's no issue. Just as if 2 people in a relationship are happy to just see each other and be monogamous.
I personally enjoy open relationships, and find that there's much more communication between myself and my partner in a open relationship than a monogamous relationship. In an open relationship you have to discuss what each of you are comfortable with, whereas in a monogamous relationship it's all just assumed.