When my husband cheated on me, I remember feeling like the world was falling apart, in the same way as when a family member died. How I got to poly is a long story...
We had grown up together, and were super-christian. Our wedding ceremony included vows that indicated fidelity in an obvious and explicit way. It didn't feel like an *assumption* of fidelity to me. I think that if he had wanted non-monogamy, he was responsible to ask first. And like Mono, I found the betrayal to be quite self-centred of him. I believed that he hadn't thought of me at all when he made the choice to have affairs.
In the present, I can see that he did think about me, and that the lying was a huge part of the feeling of betrayal. But beyond that, there was a feeling of being disrespected... of being duped into giving my rights away. For one, he left me behind in the emotional space where I stayed fidelitous for a very long time. But also, it didn't give me choices: to state my preferences or negotiate our life together, to get to know his lovers in a positive fashion, or leave until it was already a huge deal.
Ultimately, though, his habit of doing things in secrecy and without my input led to the end of our relationship as husband & wife. Though not of our friendship, or of my exploration of poly.