My partner is in a cheating relationship with someone else

My fiancé is seeing a girl who has a bf but the bf doesn’t know about my fiancé. I know that polyamory preaches ethical practices. While I would never choose a partner that didn’t have another partner’s consent, I believe it’s my fiancé’s right to choose who she sees and the girl’s right to keep it a secret, as long as it does not directly affect me. I guess I’ve made a decision for myself but just wanted to hear someone else’s thoughts.

I personally would not be okay in such a situation, but I recognize that my options would be limited. I could make it clear that I was not okay with this situation and would not stay in a relationship under these terms, but if my fiancé chose to keep seeing this person, I would need to break up with someone I loved enough to get engaged to. Which is a pretty major choice and serious compromise. Different people will make different choices about what their core dealbreakers are and are not.

Consider what might be at stake in terms of drama. If the boyfriend finds out, he might make vengeful choices that blow back on you. If your fiancé's partner feels the need to act irresponsibly or selfishly to protect herself, that also puts you at risk. Are there any personal or professional threats that would be too big to make this worth it? Consider also that STI risks are much higher when all partners are not open and honest about their multiple partners. Again, different people have different risk tolerances. Make sure you know yours.

Consider what this says about your partner as well. Are there any questions about her judgment or character that this raises for you? How has she explained this situation to you? Has she asked you to do anything you find questionable to help her maintain this secret relationship? Are you okay being engaged to someone whose values differ from yours in this way?

There's also the issue where, any time someone is part of a minority group, they are expected to represent that entire group. It's bullshit and unfair, but it happens. Your fiancé is doing the polyamorous community a disservice by acting unethically and participating in cheating under the guise of a polyamorous identity. If/when the boyfriend finds out, there will be one more person in the world with a justifiable belief that polyamorous people are immoral predators - and if my inbox is any indication, there are plenty of those people out there. If you are someone who identifies as polyamorous, you may want to have a talk with your partner about her concept of what 'polyamory' means and how you are personally impacted by her choices.