Talk about your desire to use the cage not as a need for absolute chastity (of course your wife wants to continue to have sex with you!), but rather as a way to control your sexuality so that your desire is all for her, and her alone. Present it as a proof of love, a desire for total and exclusive commitment, and that trying the cage will be a test to see if, indeed, as the sites or articles say, your relationship and love can be strengthened. At this stage, don't talk about a lasting or permanent desire to wear the cage, but rather a simple test, to be presented as a little game that you will play together.
Then, when your wife has accepted the "game", and you have practiced it over a certain period of time (at least a few days)... well there is little chance that she will want to give it up! She will have discovered the advantages and pleasures of possessing the magic key, seen that a richer relationship is possible between you, that your desire for her is increased, that you are more considerate, etc, etc. And she will soon agree to settle comfortably into a relationship she didn't know about a few days earlier.
Are you sure you want this?
A word also to the attention of those gentlemen so eager to be caged. Certainly, the cage is a powerful fantasy. But like any fantasy, the transition to reality can be brutal and even cruel: think carefully about the reality of your desire to be caged. Because consider that once your wife has discovered the virtues of the cage, there is little chance that she will want to go back to the banality of a "classic" conjugal relationship. It all depends on your wife's temperament, and it's up to you to assess her profile as a future key holder. Just be aware that while most women will use the cage as a fun or convenient additive to the marital libido, some may develop a taste for it that you never expected. These women can turn out to be real dominatrixes, with all the lifestyle that this implies for the man. You risk long periods of confinement, denial of orgasm, even imposed cuckolding, etc. Are you sure that this is what you want? Remember that if your partner decides to switch to this type of domina/submissive relationship, your whole relationship will change. Being from now on the submissive, you will not have any more your word to say. It will be very difficult for you to go back and say "Honey, we're done with the cage, I'm tired of it and I'm not interested anymore". The addictive power of the cage is strong, both for the man and for the woman who has discovered its virtues. Like all addictions, it is difficult to get rid of it. And if you want to give it up one day, maybe your wife will not want you to take it off at all. This risk exists, so consider it seriously before you decide to embark on the cage adventure.
In short, only you know your pet's personality. It is therefore up to you to weigh your chances of success according to your desires and fantasies: are they compatible with her temperament? It is by skilfully bringing up the subject, and by subtly discussing it with her that you will have the answers to your questions and an idea of your chances of success.
What if she refuses outright?
I tend to think it's because you didn't do a good job of presenting your arguments: your presentation of the cage was poorly done; your haste was too visible; you brought up the submission aspect, etc. So just let some time pass. The idea, now implanted in her mind, will continue to run through her mind from time to time, and perhaps she will go and ask about it herself when your back is turned... until she ends up bringing up the subject with you again.
As a woman, I have personally convinced several friends. Of course, it's easier to talk about this subject between women, but I always notice a very strong interest in the cage among women who have never heard of it, as long as you use the right arguments, and always avoiding strictly the submission aspect and SM folklore.
In short, there is always a way to convince a woman to test the cage on her husband, it's all a question of choosing the right arguments. This page gives you some hints, it's up to you to put them into practice if you wish.