The thing is, it sounds like in the OP's case, the original conversation wasn't "You can't let anyone else mark your body"... It was "I'd rather no one else mark your body." "Okay, no one else will mark my body, hey new partner, you can't mark my body." "Okay, I won't mark your body."
The person setting the boundary (or making the rule, if you prefer) for the new partner wasn't the OP, it was her partner. The one whose body was actually marked. The one who was told it wouldn't be, and told the OP it wouldn't be.
The other thing, though... There is nothing inherently wrong with wanting to set a structure for a relationship. Maybe it's not your thing; maybe in your world of poly, everyone does their own thing and it's no one else's business. That's fine. That's your way. For some of us, though, boundaries are necessary for our own comfort.
That isn't to say it's acceptable to control the behavior of someone who isn't even involved. I had an issue with Woody because he was apparently allowing one of his other partners to dictate whether or not he fluid bonded with me or anyone else, which was unfair, in my opinion, because he was allowing her to determine *my* sex life and relationship. The conversation, as I understood it, was her saying "You can't fluid bond with anyone else" and him saying "Okay" and then him telling me fluid bonding was off the table. I wasn't okay with that. I would have been okay with him saying "She told me she's uncomfortable with me fluid bonding with others if I do with her. I've thought it over and want to fluid bond with her. That means I would not be able to fluid bond with you, though, so I wanted to hear what you thought."
Those might seem like the same thing, and I'm having trouble articulating why they aren't... but in the reality case, *she* made the decision as to whether he would fluid bond with others, which should have been a decision for *him* to make and to discuss with his other partners instead of presenting it as "This is how it is because she said so."
(It's all irrelevant now, she broke up with him and he and I are now fluid bonded. And I have told him that if he chooses to fluid bond with someone else, I would appreciate him talking to me beforehand so I can decide whether to start using condoms with him again or continue status quo. I have *not* said he can't fluid bond with anyone else, because that isn't my call; my only choice is what I would do in response.)
Having an open relationship isn't, for everyone, about "ooh, sexy exciting times in my house with someone else, woohoo!" For me, being able to see other people is about making loving connections with others; sex is a cool thing when it happens, but that isn't the purpose. Nor was it the purpose during the six months or so when the openness *was* purely about sex. I would have considered it extremely disrespectful to bring someone else into my home, let alone fuck them here in disregard of something Hubby asked for. I *did* consider it extremely disrespectful when Hubby outright disregarded our agreement to be completely honest with each other, and lied to me about his plan to hook up with a woman he knew.
Compersion doesn't mean accepting assholistic behavior from your partner. It means being happy for your partner that they have someone else who makes them happy. If they're being an asshole, you can still be pissed off.