I can't imagine my husband telling me "no dogs". No, I don't advocate forcing my will over his, but I also don't understand how you have the right to deny her a pet.
The most important part is because this involves a living, loving animal that deserves nothing but love you both do have to be in agreement. I think you need to really consider your wife's desire to have a dog and what motivates her now after all these years. Maybe there is something else on her mind, like needing more socializing or maybe even just needing more cuddling.