"Do you really want me to make a rash decision about this? I need to time to think. Can't you give me that?"
Excuse me, but WE should not be making rash decisions here... But ONE of us already DID in secret... You have HAD time to think and WASTED it frolicing around with another man...
I have no intention of leaving my home so you can frolic around more. WE need to sort this out as a COUPLE.. it is OUR decision.. Not just YOURS. I have to decide if I want to be married to someone who CHEATS on me with ANOTHER MAN in SECRET to solve her marital problems. Pouring gasoline on a house on fire isn't going to get the occupants out safely now is it?
Stuff like that... You have to hit her HARD and EVEN... You don't want to vilify her, but you DO want to even the playing field and acknowledge that you BOTH have to think hard on this and you BOTH have reason to doubt the other. Lastly that it should be done as a couple withOUT interference from an interloping third party.