Would you be okay with this?

So my fiancé didn't tell me that one of our mutual friends was coming onto him, she said she had feelings for him and he continued to hang out with her, and let me hang out with her, and didn't tell me. It just really bugs me that he wouldn't be honest about it, I kinda feel like it's not good respect either for him to hang out and talk to her after that either. Should I just let it go? Am I over reacting ?:(