I may not be but that doesn't mean I don't abide by the rules of monogamy when I'm in a monogamous relationship. However, I have fairly strict boundaries. No, not everything is everyone's business. If someone I'm not in a relationship with asked me any number of questions, they'd get no answer. I do not automatically view it as my duty to reveal any number of complex or messy situations in my life to a virtual stranger and view it as super suspicious if anyone claims anything different. Those are typically the type of people to expect others to be very forthcoming even with sensitive information but are unwilling, or unable, to share anything equally so. And no, if I were in their situation, a zombie relationship, I definitely would not jeopardize whatever it is I'm unwilling to destabilize in my life for an unusually high level of informational intimacy with somebody who could turn out to be a nobody in the near future. Characterizing that situation in the same way that you would if the people were in a seemingly happy, stable, relationship seems more than a little insane to me. If your expectations are that high for their moral fortitude, I expect your ideology and discipline to be top notch everywhere else in your life, 0 flaws.
I couldn't care less how you rationalize it when you decide to say something fucked up to someone.