Hey and welcome,
I am sorry that you are suffering, I know this situation too well myself. I have been in the position of your wife and no, your marriage doesn't have to be over now.
I guess the discussion was heated and on your part quite desperate when you came to the conclusion that she should do what she wants. You need to sit down together again and talk about it in a more level-headed manner. Your wife got a point there when she said that the agreement was na´ve. Because you can't break off with feelings. That's the reason it will be hard for her to just follow your wish and write him off, because you normally can't stop feeling from one moment to the other.
But she needs to realize that she has some points on her agenda now as well. She can't expect you to hear about a new love in her life, clap hands and saying: Yeah, go for it! You need time to adjust to the thought, the consequences and to find out what you are able to handle and what not in this regard. You both need time. She will need some as well, because if she is still in love with you, she should aim at getting everyone she is involved with satisfied. At least this was my concern right from the start when I fell for my best friend.
The choice you have got now, is find a way to slow down to adjust to this new situation. Ask her to give you the time you need and to do it together with you. You will need each other during the steps that follow now if you decide to open up to more than sexual intercourse and allow feelings in the picture.
And a word of encouragement: I never stopped loving my husband. The love didn't diminish or felt less important to me. The new love simply added to all the love that was already there. Good luck to you.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.