One of the interesting things about polyamory versus standard monogamy is that there is no "stealing" someone away. In standard monogamy, if your SO met someone "better" you may have to worry about losing them.
But in polyamory, there is no need to steal. The only reason for them to leave you is if you two are no longer working out. It is independent of any other relationship.
There are always caveats and disclaimers, but I tend to see this as a general rule for me.