Originally Posted by Libertine
I'm distracted by desire for a physical relationship with other people to the point where I cannot function
This sounds more serious that simply wanting to not be monogamous. If your need for sex is so strong that you can not function normally, you should see a counselor. Even if your relationship with your fiance was open, that doesn't guarantee you won't have dry spells of monogamy (or...gasp...dry spells in the marriage itself!)
That said, just because you are not wired to be exclusive does not mean you are incapable of monogamy. You're an adult and are in full control of your own actions. There is no excuse for cheating.
The real question is: if your fiance wants a monogamous relationship and you don't, can you abide that? Which do you want more? If you would be willing to be monogamous to keep him, then I think you can approach the topic more slowly, knowing he may never consent (and accepting that). My hubby would not have consented to poly when we were engaged, but it didn't come up until we were well established as a couple and very secure in the relationship. We felt so secure, and trusted each other so much, that we didn't get jealous. That naturally lead into poly for us. But if it hadn't, it would have been ok.
If, however, you determine that you want an open relationship more than you want to be with your fiance, you should tell him as soon as possible so that you can cancel your wedding based on this new information. If you are definitely not willing to be monogamous and he is definitely not willing to be non-monogamous then you are fundamentally incompatible.