I love being friendly with metamours, but there have been a couple of times in my experience when I had to ask myself, "How can someone I love, love someone like her? We’re so different!" Part of the joy of polyamory is, for some people, variety. That means you might always like the person that your partner dates. But it takes a lot of stamina and emotional maturity to smile and be polite with someone that you don't have friendly feelings toward.
Some partners negotiate "veto rights," where partners agree not to date anyone their partner "vetoes." Other poly people don't appreciate these kinds of restrictions. Either way has its pros and cons. Regardless of how you choose to manage your metamours, it's something to discuss with your partners well before the situation presents itself, when everyone is feeling secure, and there is no New Relationship Energy to contend with.