When there is infidelity, it seems like most of the time it gets nasty anyway, even if the betrayed tries to keep it civil and doesn't expose. It has a way of blowing up.
Cheater doesn't want divorce, she wants you and him. She doesn't want to be known as a cheater, so as soon as she knows that you know, she may start demonizing you to all others to justify her cheating. She may already be doing so to justify it to herself or as a preparation for divorcing you. Hard to stay civil under those circumstances.
If you want to keep it civil, don't tell her you know she's cheating. Just tell her you want out, it's not working out for you, you just grew apart, you're going in different directions, you no longer have anything in common, there's no spark between you anymore, you don't feel a "love connection" like you used to, you haven't been happy for a long time, you feel like only roommates, you just don't feel any passion anymore, you think that you both would be better off apart, you can always remain friends and co-parent the kids, and you've decided to seek divorce. You can still out her to other man's wife anonymously as a work colleague so your wife won't know it was you. You can tell her you knew about the cheating AFTER the divorce is finalized, at which time you can tell her how bad you played her to get a favorable settlement if you want to.
In my case, I exposed pre filing for divorce and it had blowback in terms of my ex painting me to her family as a crazed jilted husband who couldn't move on, they circled the wagons and now have very little interaction with my son.
Doing it all over I would have changed the order to this:
- Document and evidence gather
Not expose or put OM on cheaterville
I would have done the above during the peak if her EA where she would be most agreeable and fast tracked the divorce.
My ex was checked out long ago and the push for exposure and cheaterville wasn't necessary; in hindsight I would have gone overboard alpha. There are times now the urging to expose and cheaterville seemed more to entertain TAM lurkers than to serve a positive purpose.
You can always R after divorce.