I was in a long distance relationship with a guy when I was newly poly, who had a mentally ill wife who could do no work, no housework, could barely take care of her grooming... no sex, no real marital relationship. No normal emotional relating.
I was in a grey area, but I felt bad for him, compassionate, since he'd had no affection, much less sex, for several years.
So we texted a lot, shared affection, fantasies, day to day news, histories, a bit of mild cyber sex. He was such a sweet guy. We met 3 or 4 times, had nice dates and sex. I liked him. He loved me, I was a lifeline for him. He also had a somewhat less mentally ill teen son at the time I was seeing him, who was also a huge challenge.
He was afraid to ask his wife to open the relationship, afraid she'd get more ill from that request. But he finally felt too guilty carrying on with me, and broke off.
A year or so later, he messaged me to say he had taken the bull by the horns, asked his wife to Open, she was rather blase about it, said yes, and he was now free to have other romantic sexual relationships. He asked if I was still interested. I wasn't actually, at that time, interested. I'd moved on. But he messaged from time to time and I was glad to hear he was dating some other poly people.
So, that's just my experience from the other side.