Lesson learned: make a deep assessment of your partner's intensions. If there is slight hint of lack of authenticity, better not give Poly a try and/or wait until later.
Here in this forum, and also in books about poly relationships, honesty and openness about feelings is something very remarked because quite simply you can't expect the partner to read your mind, even more so when you're reaching the boundaries of what each considers acceptable or significantly undermining.
That's something I've been practicing more often lately with my main partner. He knew I wanted a poly relationship when we started some years ago, but only now is he understanding the full consequences, that I might want to love / have sex with someone he doesn't like or he's jealous about, or he's feeling insecure about our relationship. So I'm letting him know more than ever that I love him and he's special to me, and I often ask him 'How do you feel?'. 'How do you feel' expecting an honest reply. He feels he wants to be with me and he'll go through the trouble of getting fully accustomed to an open relationship. But if at any point he's feeling down or has doubts, I'll spend time with him or talk with him if needed.
There is little chance of making a deep assessment of your partner's intentions if they're not being honest / don't see the importance of being honest about something as touchy as this subject.