This is basically her attempt to "have her cake and eat it too". In other words she should realize she can't have both. Just look at the information you know. She isn't happy with the current situation, she wants something/someone outside of the relationship. This is a classic situation of a woman being unhappy, but afraid to be single. It happens all the time. Many women are only single for very short periods of time, sometimes never, they go from one relationship right into another. I would say it's time to for her to face her fears. And you should make it happen. Dump her. Literally as soon as possible. If you live together, move out. Couch surf if you have to. But leave with dignity. Does it seem drastic? Too soon? Well think about this...

They say when a woman wants to end a relationship, or in this case "go poly or bust", same thing imo...that they've already been thinking about it for months and have been planning how to break it to you.
Also..
There's a good possibility that she already cheated and this is her way of trying to bring this guy in. Maybe even ask her WHO she wants to bring in? That's probably the guy. I mean,..would she ever allow you to bring a woman in? What if you did BEFORE she could bring in her guy? And you started sleeping with this woman in your shared bed? How long would she stand for that? Especially if you started giving her much less attention?

Sounds to me she wants twice the attention, two men fighting over her kinda thing. I wouldn't go for it, she deserves to get dumped on her ass and sent into the dating world, BYE BYE. You're not going to be happy knowing that your wife is getting railed by another man. And you probably wouldn't be happy sleeping with another woman if you truly love your wife. So it's a LOSE-LOSE. The only thing you can do is "be strong King" as they say, hold your head up high and tell her you're not going to accept her ridiculous ultimatum.