I take issue with the poster above who says, you are already cheating on your wife since you think she wants you to have feelings only for her. People can't control their emotions. You can't say, 'I will not be jealous' or 'I will not develop romantic feelings for this woman' or 'I will be happy as a mono'. You can control your actions tho, and you are doing so. Good for you.
I think that to betray someone, you have to DO something vile; you have to DO something more than thinking something or feeling something.
When my wife and I came out about being poly, I was a bit surprised by who was OK with it and who seemed to be bothered by it. A couple of the people who I thought would be closed minded were OK with it, and a couple I thought would be cool were a little bit judgmental.
If you talk to your wife about this, she may surprise you and be calmer about it than you fear. Or it may be close to your worse case scenario. However, right now you can talk about these things from a superior position than if you had done something vile and already betrayed her. You can say, 'I have these feelings' and then be totally honest when you say, 'I have done nothing to be ashamed of'. Telling her is just being honest about how you feel - which is courageous.
Right now you have something very good, and talking to your wife will risk that. If you don't tell her, you are hiding something important about your relationship from her and setting your self up for a certain amount of unhappiness down the road. But quoting the Larry Niven parable, "I might die, the King might die, or the horse may learn to sing". (The horse singing in this case would be that your wife brings up wanting to be poly.)
Ultimately it is your choice. Only you know how you feel, and you can estimate how your wife will react better than we can. Only you can estimate the risks and advantages of the various acts you can take.
However, what ever you choose to do, I salute your courage and hope things work out well for you.
Warm regards, Rick.