Summary
When a man explicitly sets boundaries, he is using an ultimatum.
Ultimatums don't work. A man can never say "You MUST do X" or "You MUST NOT do Y" and have it be effective.
This is different than not tolerating her BS and being willing to walk.
Boundaries should be implicit, and a man should put his foot down over certain actions as they happen. He should not set rules for her future actions.
- Good (Implicit Boundary): If you want to date him, that's fine, but I need to cancel our plans for this week.
- Bad (Explicit Boundary) : You are not allowed to date other men, or else.
In one case you are removing the reward for good behavior should it cease. In the other case you are threatening punishment for bad behavior should it occur. Both cases imply that you have set a boundary that she can't date other men.
In one case you're always happy either way, you're happy with her or without her. In the second case you are implying your happiness is based on her behavior, and that you'll be upset if you don't have her.
These are not the same thing and I think this is where the disagreement lies.