Welcome to the forum.
Fascinating for me to read your post. It made me think much of my dad. I have heard him (not very often, but oft enough for me to remember) complain about monogamy, that it's the price to pay for having the girl. I think that part of him is pretty happy for me. I know he's enjoyed seeing me rebel against tradition throughout my life.
I also thought, as I read, that if I were your wife, and I heard the statement, 'I don't believe in monogamy,' from a man who stood up, presumably in front of friends and family, and took vows with me, I would hear something along the lines of 'I'm unhappy with this marriage and thinking of not being married.' I would tend to hear that very loudly.
If you really want to live poly, honesty is a really important thing to practice. Why not practice that where you are? If you can't be honest with her, how are you going to be honest with the next one (or two or three)?
So ya, if you're looking for opinions, I think you should tell your wife all of who you think you are. Who you are is different than how you behave. You can tell her you're poly and still choose to live monogamously with her. You can still take up a serious hobby or sport. But you'll be honest, and you'll know that she's with you because she loves who you are and not some fantasy you're projecting.
However, the method of telling is important. If someone said to me they were 'living only part of my life,' I'd be very unhappy that person spent 13 years with me, faking it.
I encourage you to do lots more research about telling, opening up, etc., before you have that conversation with her. There is tons of information in these forums. There's a sticky thread with good books, and though I haven't read it, I see folks recommending Tristan Taormino's 'Opening Up' for situations much like yours.
I think it's great you've found these forums, and you're able to write out many years' suppression!