"if you weren't here, she and I would be together"
Well, sure him and her and her spouse and her two friends with benefits.... I doubt they'd be as together as he thinks. If she would ditch her other partners to "be with him" she isn't particularly very good at being poly. If she is aware of his cheating, she isn't particularly very good at being poly. It sounds like he's not really operating in reality.
I am really sorry about this introduction by him into an open relationship. I suggest you (and he, if you still want to be with him) read some of the books that are more geared towards ethical non-monogamy of a polyamory variety (not that ethical slut isn't kind of but...the book resource list has a lot of other great books that will help you decide if honest polyamory is something that you could either embrace for yourself, or accept in him). Discuss with him what he is willing to do to make this a positive experience for you.
I second mostlyclueless' post too.
Happiness will never come to those who fail to appreciate what they already have.