You found out that you have been a participant in infidelity.
Before I dive into this tricky situation, I want kiss the ass of my favorite podcast host, LGBT activist, and sex columnist, Dan Savage. I'm bringing him up because, based on what I've read and listened to, this is typically the only topic we differ in opinion on.
9. It is your business.
Another common argument I've heard about this subject is that somebody else's relationship is not the "other woman's" or "other man's" business. If you are inside of me, your marital status is my business. It's expected for one to disclose whether or not they are committed to somebody, just as it is expected to disclose whatever STI they may have. That is why one can file a lawsuit against a sexual partner for transmitting an STI if they knowingly withheld that information.
Take some personal responsibility and stop being a shitty person.