Would you consider it a perfectly legitimate arrangement if one end of the “V” resents it and is unhappy?

That's just called cheating, not polyarmory.

Mind you, I've been in this setup you describe for a long time. My previous partner had female partners on top of me after ~7 years of only having me, and while I was friends with some of the women - good friends with one, even - I wasn't ever "close" to most of them. Worked perfectly fine for me.

And this wasn't a short thing either, we were together for ~10 years after that point, and the longest "third" partner was for 6 years.