okay
first, i'm not in any relationship right now and have never been in an open one. if that option was open to me in the future, i wouldn't rule it out. my decision would have to be based on my feelings and situation at the time. basically, i'm talking out of my ass right now.
second, you don't need to explain to us or justify open relationships, we know all abouts em
last, your problem. what can i say? your stability depends entirely on your relationship with your SO, or that relationship compared to the other people they may be seeing. if you feel your bond is strong, there's not much to worry about. the stability of any relationship is entirely dependant on the people involved. its really the same with a regular relationship, except its easier to phase out of an open relationship, while you have to just plain jump ship in a traditional one. there is no easy answer. maybe, if you feel uncomfortable with the uncertainty of the open relationship (you wouldn't be posting this if you weren't, would you?), you should sit your SO down and have a talk.