I lost my best friend because of ENM
We had been best friends since we were 19 (in our 30s now).
I began my ENM journey and read the ethical slut in my early 20s, so he has always known that this is who I am, and he never had a problem with this (even though he is the most monogamous person I know, still with his high school sweetheart).
The problem arose last year, when I started dating his childhood friend (Red).
Suddenly what I was doing wasn't ethical anymore. He said it wasn't fair, because I have a long term partner and Red had nobody (besides me I guess). He said that I should be mature enough to not follow a crush. He said that letting Red decide if he wanted to date me or not was not a valid option because Red is the kind of person that follows other people's choices.
Well, I didn't stop dating Red just because my friend said so, and he stopped talking to me. Completely avoids me. The most that I got from him (a year ago, 4 or 5 months after the initial conversation) was "yes, I'm angry with you, no, I don't want to talk about it."
That's it. More than 12 years of friendship, gone without a conversation or an explanation.
I felt extremely guilty for months, a part of me thought that I was really doing something unforgivable and terrible to Red: why else my best friends of more than a decade would stop talking to me so abruptly?
I'm still with my long term partner, I'm still dating Red. He is happy with me and the arrangement. On the rare occasion that I meet my (ex) best friend, he can't even look at me. And I'm still not sure what his real problem is.