This is incredibly sad, but I have to say, not a complete surprise to me.
There are lots of posers in both the poly community and the bdsm community. They say one thing and do exactly the opposite - they don't even come close to living up to the goals that they are so willing to tell everyone about and expect them to stick to. Trying to identify them and avoid them (they are often quite predatory) is an art.
One of the other descriptions for poly (and swinging for that matter) is "responsible non-monogamy". What he did to you wasn't responsible, nor was it open and honest. In short, this is about as opposite to poly as you can get. They both cheated. Clear and simple.
I hope you can see from reading around here that there are other, far better, ways of doing this, where everyone can be happy and content. I'm really sad that this didn't work out for you.
"Listen, or your tongue will make you deaf." - Native American Proverb