From the few married couples I know in open relationships, this area is covered by the ongoing conversation and agreements they have regarding the details of their polyamory. They don't carelessly engage in random relationships, and they partner only with people they know very well. Usually their extra-marital partners are long-time lovers that both people in the marriage approve of. I don't know how they manage the emotions of polyamory, but I do know that they are far more cautious and slow-moving than most of the single people I know, and they sleep with fewer people than my single friends. Of course, "polyamory" covers a wide range of lifestyles, but it seems people have the impression it means having relationships with gobs of lovers at one time. In my handful of observations, polyamorous folks have fewer partners than serial monogamists.
Edit: Oh, I just realized I mistook "open" for "polyamory". I suppose open is a subset of polyamory, but I don't think my answer is quite relevant to your post in that light.