I was interested in how Mindsin would suggest you make it "your" boundary. A personal boundary is not about controlling the other person.
First question for 3kids - How did you find out about the weekend, and what (if anything) did you say to your W?
He can start by saying something like, "Taking the kids to spend a weekend with the OM made me feel really disrespected a father."
She may disagree. Also, enforcement will be difficult since he didn't set that boundary earlier.
In my W's case, I said pretty much the same thing to her. She thought about it for a while, and replied, "Yes. You're absolutely right. That was wrong of me."
M: 15 years BD: 6/25/14 EA/PA: starts 5/14/14 11/30/14 - A ends 5/15/15 - D is finalized. 11/28/15 - Start of new LTR with a wonderful woman (and still going strong)!