There's a lot of information missing....
By "pretending" to be mono, do you mean he tells you he's being monogamous and then he turns around and cheats on you?
If that's the case, then he's not poly--he's simply a cheater, a bad monogamist. Trying to excuse his cheating by saying he's "just poly" is so much bullshit. Simply fucking around isn't poly, particularly if he said he wasn't going to do that.
I agree with YGirl--it appears there are a whole bunch of issues that need to be addressed in your marriage and polyamory has nothing to do with those.
It sounds really painful from here and I have to wonder how it's lasted so long. I hope you can find some way out of the mess in short order.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.