I'm not against polyamory, but I find it never works out when somebody decides to open up a previously monogamous relationship.
A (former) close friend of mine got engaged a few years back, and then his fiancé brought a third into the relationship and they became a throuple. That relationship lasted maybe another year or two, until she called off the engagement several days after his mother passed away, and left him for the other guy while he was grieving.
Found out sometime later that the third she brought into their relationship was actually her cousin, so there was definitely some Sweet Home Alabama crap going on.
Another friend of mine tried to commit suicide recently after getting dumped. Her relationship with her boyfriend of ~2 years had been rocky to say the least. They broke up, got back together, etc quite a few times, but the most recent breakup was after he brought a third into the relationship, then allegedly found out she was texting a guy and dumped her on the spot.