And thanks for your supportive advice, I really appreciate your taking the time to give my situation some thought.
You are right in that she didn't realise that my partner Z doesn't just sleep around with people willy nilly. She was cool with that after a time. I think at nineteen and in a 3 year old relationship herself she projects her emotions onto me.
It was when I tried to explain a triad relationship that she lost it. I hope I don't offend anyone when I say she screamed at me that she didn;t want a "Lesbo mother". I found this attitude disappointing in my own daughter as some of her best friends are Lesbian and bi. She said that's fine for them but not for me. I was initially upset but now I see it as just another example of her wanting me to be something that I'm not. It has probably come from her father, he was very similar and it's interesting that these threads continue even after the marriage has ended.
Even though at the moment I'm pretty much on the mono side of poly I find having polyamory in my life a great personal growth experience.