You’ve said that your poly friend has been in relationship with her guys for several years, which means that they have inevitably had both good and bad times together. You can’t be in an intimate relationship of any kind over time without that happening. I’m also going to guess that they have more nuanced relationships than just “the emo guy” or “the hiking guy.” We are talking about people here, after all.
Before there was patriarchy, marriage, and men with multiple partners, there was a time when everyone had multiple partners. It’s what 97% of human history has been like. It’s not taking on masculine traits to the negation of feminine ones to be starting to revert back to this state. As long as there is honesty, transparency, good communication, reduced jealousy and possessiveness -actual ethical non-monogamy practices in play, people should be able to co-create the kinds of relationships that work for them and feel good about it.
My husband and I opened up our 20+ year marriage about 5 years ago and it had some bumps in doing so, but ultimately we’ve dismantled a lot of unconsciously internalized crap about what it means to be a husband/wife, and we are both a lot happier. We are now partners and not roles that were written by someone else that we must fulfill. Although I think it’s theoretically possible to do that in a monogamous relationship, I also believe it would be a whole lot harder. We’ve always had a healthy and pretty egalitarian relationship. I’m pretty self-aware and yet I still didn’t truly grasp how much of my true self was being repressed in monogamous marriage. It took stepping outside of that to find who I actually really am, and who my primary partner actually truly is. We’ve both grown immensely as individuals and as a couple. It took real effort to get here, but most of the work had to do with dismantling non-serving monogamy paradigms.
Obviously, people should do what makes them happy and that may very well be monogamy, but my friend E. L. Byrne has also written extensively about how she found a wholer, more authentic self when she became polyamorous. Here’s her story on intimacy that speaks to a lot of your questions.