Yes, I've been there.
I had a friend of 3 years. He had a more conservative/religious upbringing. He believed in monogamy. He wanted to get married and have kids soon and we both recognized that we weren't life partnership material. We were good friends, mutually supportive of the other's relationship attempts for those 3 years.
There had always been some mutual attraction. We started sitting closer and closer on the couch. Cuddled. He got "handsy." I'd fallen for him despite some misgivings - those reasons why we'd said we'd just be friends when we met.
I have a hard time calling it a relationship. Entangled is a better word for it. I tried to be more vulnerable with him. Create a deeper emotional connection. Integrate him more into my life. He wasn't going there and he wasn't a great communicator. I got lots of mixed signals. It was frustrating and oddly embarrassing.
We would get entangled, then I'd feel used and disconnected and call it quits. We'd stay friends. We'd get closer on the couch again. Get entangled. That was the pattern through four "break ups."
We had a lot of chemistry, but very little compatibility.
My friend/lover/whatever-he-was justified continuing to make advances after I'd ended that part of our relationship (over and over again) because my being poly meant I was "promiscuous." His word. Frankly, I think he had feelings for me, he could be very thoughtful and tender, but he didn't want to actually be in a poly relationship. To him, that would be wrong.
We didn't talk for a year and a half after the last break up. We're talking again now. He recently got married and I'm happy for him. I learned from the experience.