On a more general note, I really don't see how this topic fits into the "Morals" forum - I consider it a topic for Social Science.
There's a lot of interesting questions to ask about open relationships - how likely are they to succeed, what are some common problems and benefits, which personality types are most likely to enjoy themselves in one, are there any demographic data about success rates/average durations, if and when they end, is the break-up typically more or less painful than in a (nominally) exclusive relationship, and many others. Those are questions for sociology and psychology.
The question of "right" or "wrong" doesn't enter the picture if all involved know what they are entering, and choose to do so.