I think the difference between a boundary and an ultimatum is the motive. A boundary is a limit on the behavior I will expose myself to. If I have to leave to protect myself, it's still a boundary. An ultimatum is a threat designed to force someone to change their behavior. The outcome desired is not self-protection, but a change in someone else. Boundaries require acceptance that the other will behave as they will and my reaction has no power over them.
Having said all that, I could not stay in a relationship where I was detached from my partner for most of the time.
L