As you know, she is playing with fire, and she could burn the house down. but at least she is warning you so you can get out. That is thoughtful of her.
Before I married I liked to have 2 boyfriends at all times, one local and one long-distance. That way if one broke up with me I still had the other for emotional support. So, when I married my local boyfriend, Mr. Longdistance was still in the background. I thought about him often, even while I felt in love with my husband. I felt that I had the ability to love them both for themselves. But over the years, the tedium of marriage set in, and Mr. Longdistance started to look better. But he had gotten married too. So we kept up the talk, but didn't do much about it.
This was not a good thing for my marriage. I was not more than 3/4 committed at any time. And though I don't think my H knew exactly what was wrong, he sensed my emotional distance. Eventually as a self-defense mechanism, he had an affair. And lo and behold, that was a crisis. Because it wasn't me leaving him, it was him leaving me! And I wasn't prepared for that.
I can tell you with certainty it is not a good thing that your wife wants to have an open affair with another man. But your chances are about 50/50, that she may decide she prefers you if you simply let it be a dramaless event.
Truth is, if she is really committed to you and your marriage, she wouldn't do it. The fact that she is doing this shows she is not really committed.
