The infidelity doesn't come into play in our state.
FWIW, even in jurisdictions where there is fault, infidelity is very rarely used as it is difficult to prove legally.
Unless I want to take everything to a spiteful place legally,...
Filing to divorce your spouse who openly (admits to sex, discusses with her mother, sees regularly) has a relationship with another man is not being spiteful, it is being reasonable. You can D without being spiteful.
Now, I would say that openly participating in an A with an OM, while still maintaining your M relationship is spiteful, as well as selfish.