I agree with leaving...for one reason: inconsiderate bullshit.
Even if you were in, say a V, your relationship, your actions WILL affect the other person you're not directly connected to. Infedelity due to a very fast New thing I would consider forgiving. For revenge? Fuck no. These people have some bad habits about payback. Revenge doesn't work in poly, period, the end.
I'm sorry you've found yourself in this position. Eject, before you go down with the plane.
“For an instant I think I saw. I saw the loneliness of man as a gigantic wave which had been frozen in front of me, held back by the invisible wall of a metaphor.”
― Carlos Castaneda, Journey to Ixtlan