Relocated the family and he dropped the bomb

[TimeHeals]

I concur: "I will not live in an open marriage" is a boundary.

How do you enforce the boundary? You ask him to leave if he doesn't end it now, and if that doesn't get him to either move or end it, then you hire an attorney and start making things legal.

Assert your boundary. If the boundary is overstepped, execute the consequences: asking him to move out, hiring a family law attorney, etc. You don't threaten the action. The action is a consequence for not observing your boundaries.

Boundaries are about protecting yourself from harmful behavior of others, not trying to change others.

His behavior and choices are his. You aren't trying to change him. You are asserting your boundaries and letting him choose to either observe them or deal with the consequences of not observing them.