I'm not sure if my situation applies... In my case I'm poly and dating a poly man. So feelings and relationships are allowed with us, and certainly not restricted with outside partners, although neither of us has been very lucky finding anyone else so far.
However, I briefly had a FWB. There were rules that we were not to be in a relationship, but they were places by him. He was (and still is) mono, and did not want a relationship with someone who was poly. As a result, he wanted to make sure he didn't get attached, and didn't want me to get too attached either.
Not much happened as a result. The impossibility to develop a relationship with him made me less eager to spend time with him in a sexual way, so we mostly maintained our previous (and current) status as just friends, with a bit of flirting and sex, but only a couple of times over a year, so nothing major.
Then he met someone he wanted to be monogamous with, as we both knew would happen, and the friendship lost its benefits.
I can talk more about my experience if you want, although it's not exactly what you asked for.