Originally Posted by london
This is part of the reason I avoid these types of relationships with metamours.
In some situations that is almost impossible. For example, with my ex, she was (and still is) very much entwined with my friendship group. My boyfriend and I are good friends with the same people. Therefore, whether he wanted to or not he would be spending a lot of time with her (unless he just stopped spending time with his friends at any public events). Luckily, he adored her and actually prefers the idea of knowing his metamours.
So my point is, if you and your partner(s) do a lot of dating outside your social circle, or you have different social circles, then avoiding/not getting to know metamours is definitely an option. When they're pretty much everywhere you go, it's difficult, and probably not very healthy to avoid them. These are probably the circumstances where break-ups become most awkward regarding post-breakup friendships. In my opinion anyway.