Here is a classic example of a woman attempting to have her cake and eat it too. She eventually deleted the post because of the negative backlash in the comments.
Essentially, this woman thought it was a good idea to bring up the idea of an open relationship to her boyfriend of 5 years. According to her, he is "amazing" i.e. good job, great sex, good family, etc. She just wanted to have purely physical sex with other men, but it would go "both ways" as if that made it any better.
He knew what this meant, that she already had a guy (or guys) in mind and was looking for an excuse to cheat. He breaks up with her on the spot and she apparently is "heartbroken" and doesn't understand why he would end a 5 year relationship over something "so small."
Of course, the reasoning eludes her completely. Not once does she think of how he is feeling, only that she has been crying for days and is completely upset. She doesn't think that to him, her wanting an open relationship meant that it was only a matter of time before she cheated.
If he said yes, someone else would be plowing his girlfriend. If he said no, it would cause tension and eventually she would fuck someone anyway. He did the only thing he could and dumps her. He completely cuts contact and even sends his friend to pick up his stuff so she can't pull any post-break up mind games on him.
She tries to justify it by saying as a response to one of the comments, "I just wanted to talk about it. If he was into it, great. If not, then also great." Again, she doesn't understand that the second she mentioned "open relationship" it was over. No one can trust their girl anymore after she mentions that.
Bottom line: Your girl may or may not cheat (or want to cheat). If she does and comes to you with some bullshit like this, cut your losses and get out.