Your wife loves being married to you, and does not wish to have sex with other men because it may damage your marriage together. You are the only person she has had sex with, and she has probably never been told that there are other men with penises bigger than 5 inches who can make her have EVEN BETTER orgasms than you give her.
You can give your wife permission to have sex with other men - but do not badger her with pressure to do so or she may get angry one day and throw pots at you.
You need to gently tell your wife WHY you would like her to enjoy sex with other men.
She might think you are getting tired of her (already), and are using it as an excuse to fuck other women you have your eye on. She may be wondering which women you are meeting behind her back in secret.
She will get suspicious of your reasons for her to have sex with other men - unless she can get into your mind and fully understand why you won't mind other men seeing her naked like eye candy, and kissing touching every part of her body, and maybe accidentally getting her pregnant.
Once your wife has been naked for another man, she may become addicted to it, and later on you might get jealous of some of the men she dates/mates with.
Your wife must feel convinced you love her completely, before she can take a risk that you may be "testing her" to see if she would become a slut if she got a chance.
If your wife knows she has permission to fuck other men, then she may discuss it with her family or friends to see if they agree she should. It may be a few years before she may actually find suitable men she feels comfortable with.
It is possible for a wife to fall in love with her lovers, and to feel less in love with her husband - and it is not her fault. If the other guy gives her powerful orgasms, her body may want her to spend more time with him than you.