Of course, I've had great experiences and will have more. But the key to them going well was the intention.
The real question is, should you "believe" in monogamy in relationships. Why should "relating" with one person sexually keep you from relating with others??? That notion is a result of social engineering, not nature. People would rather question themselves, their character, their self worth than question the obviously flawed system because that's all they have been given to work with.