Emm's example is good.
When I set boundaries, they're about me. They tell the people in my life what kind of behaviours they can expect from me if I notice certain behaviours from them. In the extreme, that can mean "If I notice you lying and cheating on me, I will leave you."
When I set rules, they're about you. They dictate what I'm allowing and forbidding you to do. Of course, this requires that you agree to my rules, since slavery is illegal.
In your case, you might have a personal boundary that says you can only be in mono relationships. That doesn't take away his right to have polyamorous relationships, but it does mean you'll have to be prepared to leave if you can't handle it. That's different from telling him he's not allowed to have polyamorous relationships.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."