Soooo this may not be a popular opinion, but as long as you are being honest (and it seems you are!), I don't think it's really your job to be someone else's conscience, you know? It might sound crass but I think if someone cheats (however they define "cheating"-- for sake of argument, cheating = breaking whatever rule, however mundane) on their partner, the person to blame is the person who's actually breaking said rule. Now, you might still feel bad because you "helped" the person cheat, but in my experience, someone who is going to break rules will do it regardless of whether it's with you or someone else. The real issue is that person needs to rework their "rules" to be more in tune with their own relationship needs. Sure, you can be respectful and, for example, not talk to X because Y told her she can't (which I think is some controlling, slightly creepy and maybe-leaning-towards-abusive bullshit but whatever, haha!). But if she initiates talking to you and you respond, I don't think you're in the wrong.
Tangent: my brain can't even process emotional "rules" like "don't fall in love with anybody else". Even if you're monogamous, feelings may occur, but the "rule" is generally to not act on the feelings. Which, to me, is way easier because you KNOW when you cross that line. Emotional rules are so nebulous, and by the time you realize you're breaking them, it's too late. If you're opening yourself (and relationship) up to new people, falling in love is pretty much inevitable at some point. I was in a poly situation like that, where my partner's wife told him he couldn't fall in love with anybody else, and if he did, he had to break up with her. Seriously, this goes against like every aspect of human nature (which ya know, poly is supposed to be all about NOT denying our nature). I told him I had real feelings for him and he said he felt the same, but we didn't want to break up because... duh. lol. I knew I was breaking his "rule" and disrespecting his wife by doing so. But, on the other hand, this was not MY rule, so the burden was really on him and I don't feel guilty about it.
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Tangent: my brain can't even process emotional "rules" like "don't fall in love with anybody else". Even if you're monogamous, feelings may occur, but the "rule" is generally to not act on the feelings. Which, to me, is way easier because you KNOW when you cross that line. Emotional rules are so nebulous, and by the time you realize you're breaking them, it's too late. If you're opening yourself (and relationship) up to new people, falling in love is pretty much inevitable at some point. I was in a poly situation like that, where my partner's wife told him he couldn't fall in love with anybody else, and if he did, he had to break up with her. Seriously, this goes against like every aspect of human nature (which ya know, poly is supposed to be all about NOT denying our nature). I told him I had real feelings for him and he said he felt the same, but we didn't want to break up because... duh. lol. I knew I was breaking his "rule" and disrespecting his wife by doing so. But, on the other hand, this was not MY rule, so the burden was really on him and I don't feel guilty about it.