maybe people don't object to a metamour as much as they object to trying to figure out what kind of relationship they are suppose to have with them!
Before I started dating O, I was friends with the two of them and didn't know that he was interested in a poly kind of thing. I felt slightly awkward around A (mainly because I had some feelings for O) but overall we had a good rapport and felt fairly comfortable. Then when I officially started seeing O it seemed like we didn't quite know what to do with each other (maybe it was just my perception) but it was definitely a transition and I feel like we've found a good rhythm for now of how the puzzle pieces fit together. But it was a bit awkward there for a little while.