I would want to know because I would end the relationship.
14 years ago, I discovered my now exh was having an affair. At the time, I had no idea and discovered it purely by chance. I also did a hobby and two of the women there also worked with my exh. I was quite friendly with one - we'd meet up and have lunch/coffee occasionally and knew the other to chat to at the hobby.
When I found out, I told the woman I was closer to, not realising that she already knew (he had an affair with a woman from work) and she just said, "Thank God you know," and went on to tell me that they had both lost a lot of respect at work and that she and the other hobby woman hadn't known whether to tell me or not. What had made it difficult was that they both worked closely with him and he had a reputation of not being a very nice person at work. It would have come back on them.
I completely understood why they hadn't told me. I approached the other woman at the hobby whose face upon hearing me say, "Have you got a minute for a chat?" Told me everything I needed to know about how she'd felt about knowing and not sharing. I told her I knew and that I didn't hold it against her for not telling me. It was fine. Again the relief on her face was palpable.
But I would always want to know. I'm not staying in a relationship with someone who'd cheat on me especially someone who would put others in such an awkward and uncomfortable position. He was arrogant and compassionless to everyone.
3 weeks after I'd kicked him out, he told me he still loved me and asked could we work it out. I laughed and asked if the OW knew he was saying this. He said no and I pondered over who he was actually cheating on and with whom now.
He told me he was going to 'make a go of it' with her now as I'd said that and he felt he needed to 'make the best of a bad situation'.
Why would I want a such a weak and spinless individual in my life?