Eh, one-on-one partnering is kind of the social norm and it's okay to have/set that boundary for yourself or expect that for others because it's socially implied that you could be crossing that boundary for someone else without their say (eg. the boyfriend). If someone has disentangling to do, they need to do it on their time, not yours. Scruples aside, it sounds like way too much problem and effort to be a component of someone else's breakup, and in someone else's shoes, I wouldn't want to inflict a hypothetical breakup of my own on a romantic interest.
If she mentions they're not exclusive or that they're poly, then the boundary's changed and it's open swim, right?
Never mind the wrath of jilted lovers, I don't really trust other people not to go on a violent tirade when they're crossed. I mean, I don't really trust other people to take their own health or others' health into consideration, de-escalate a problem, understand driving laws or things like right-of-way, or even vaguely to do the right thing, but that's kind of beside the point. People get shot dead for less, and as a small man, I'm not going to come out on top if someone comes at me, arms swinging.
Crushing on someone you're not supposed to sucks though. Been there a few times. It's difficult to choose sometimes how we feel, but it's how we act about it that matters, and you seem to have that locked down pat. Hah, now we're all dwelling on it for you, eh.