I forget exactly how we got on the topic Thursday night, but the rule in Frisbee and I's relationship that he can't fuck people in my family came up. Actually, it's more one of those, "not unless we really talked about it first," rules. And before you ask, no, I don't have a similar rule with Now-hubby. Although in that case it's probably a rule more along the lines of, "I don't think this should have to be said out loud, but I reserve the right to be angry if it happens because I'd still be uncomfortable with it."
Rules are a tricky subject. I think it's easy to get on the slippery slope of things that make you feel insecure or uncomfortable and put up barriers against all those things. In my opinion, it's another fantastic hold-over from the way you're supposed to think about relationships. i.e. Get together with one other person and then just be completely and 100% blissed out at all times about how perfect everything is.
So in monogamous relationships the barriers are the relationship themselves. (I had a mindfuck a few weeks ago at about a  when I mentioned to Now-hubby: "Why do straight people always complain about how unhappy they are in their relationships? Is that supposed to be how it's done?" His reply: "Why do you think they call it an 'institution?'") This exclusive relationship is set up and it's just understood that anything outside of that relationship isn't allowed anymore. Literally...anything.
In poly those barriers are set up with rules. Things are so much more open and honest than those silly straight people. And since within the relationship(s) you can talk about literally anything, it's really easy to create a list of all the things that you don't want your SO engaging in.