Serious question: What is the difference between giving an ultimatum, and telling them honestly that you can't stay if they _____?

We're always told not to give ultimatums in marriage, because it breaks the integrity of the union. And I understand that. But where is the line? If you are already married, and something arises that wasn't in play before you committed, what are you supposed to do?

Every person has boundaries and limitations. If your partner is considering doing something that will cross an irreparable boundary, you warn them, right?

Is there an appreciable difference between "If you do XYZ, I'm divorcing you", and "I'm sorry, but if you decide you must do XYZ, I cannot stay married to you and watch you do it"??