It's time to file for divorce, dude. Women can sometimes have casual sex, and keep it light. But most women tend to form emotional attachments to any guy they have sex with, even if they have other motives for doing it.
A marriage can (sometimes) survive a man having sex with another girl, because for the man....it can really mean nothing at all. Just sex.
A marriage rarely survives a woman having sex with another man. By the time a woman has decided to open her legs to someone other than her husband, it is guaranteed that she has emotionally "checked out" of that marriage.
Or let's put this another way...
Your wife is suggesting that she should f*ck another man. That means she is already willing to trash your marriage. So what would her actually having sex with the other man accomplish? It would demonstrate she's even MORE willing to trash your marriage.
What you need to be concerned about is not whether she actually has sex with another man or not. What you need to be concerned about is that your marriage is DONE. The only way forward for either of you is divorce now.
You can try to save the marriage if you want, but that will just prolong the misery. Your wife is not on your side anymore. You will be divorced within a few years, at most. For your own sanity, end it as fast as possible.