Yep. I've heard it said that one is not really polyamorous unless they can find joy in the joy that their partners experience with their other partners (i.e., compersion).
I would actually alter that to say that one is not really "practicing ethical polyamory"...... And, that is because there is a strong argument to be made that everyone is poly at some level - that is - everyone experiences or has the capacity to experience romantic attraction to more that one person at some points in their life. I would guess that almost every adult in modern western civilization has experienced that at some time in their life - certainly sexual attraction to more than one if not actual romantic attraction. It is the human condition - so, no, you are not an awful person for having these feelings, only human.
The key issue in practice, however, is whether one acts on these feelings and engages in more than one romantic/sexual relation at the same time. Polyamorists make the choice to do so - monogamists make the choice not to. Neither is right or wrong - although poly may be more natural, and a strong orientation for monogamy is most likely almost always a matter of cultural conditioning. Polyamory is better thought of as a practice or lifestyle choice rather than as orientation - since virtually everyone experiences multiple romantic attractions at some point. (Mentioned in this thread already - "Sex at Dawn" is an excellent resource on this subject).
Having said this, in marrying your wife you most likely established a contract for monogamy with her (most people marry with the expectation of monogamy). And while you can certainly renegotiate that contract if you are both willing (and it sounds like you are in that process perhaps), she is under no obligation to do so. And, if she refuses, you obviously have the choice to terminate the contract, be content to remain a practicing monogamist (even if not in spirit), or go the unethical route and cheat.
I would suggest that although you say that she is very mono, that you encourage her to date as well (and it is a well established truism that poly women have a much easier time finding dates than poly men) - it will make it much easier for her to come to terms with poly, rather than just trying to find a way to be "ok" with you having another partner. And, who knows - once she thinks about what it would be like for *her* to have another partner, she might give it stronger consideration.
And - welcome to the Forum!
Al