Vasectomy - if I don't want one an my wife is threatening divorce, what are the options?

I am a 31 year old male with a beautiful wife, a 3 year old, and one on the way. My wife is done after this one an wants me to get fixed. I wouldn't force her to have a third but I wouldn't be sad to have 3 either. I dont know that I am comfortable getting fixed. I said that if anything ever happened to us or her, there could always be the chance that I would want another one. Am I really being the incensitive, conceided *** she says I am by saying that? Isn't 31 young to be doing something permenant to my body if it's not something I want? Any marital advice is welcomed here.