Can I join the chorus agreeing with @monkeymonkey2010? 
Now she's either bullying you to 'allow' her to cheat openly - or push you to ending things.
this was exactly what my ex-wife did with me. When DS1 started school a few of the dads started flirting with her on the school run and wanted to play the field. The only problem was she was married to me. Next thing she's announcing she never really believed in monogamy and wants an open relationship - except, I kid you not - she needed "emotional connections" so she needed to have full-on relationships. I, however, being a man obviously only needed sex, so I would be restricted to one-night stands.
There were no other problems in the relationship she could speak of. Our sex life was good and I'd been at pains to be creative and (successfully) satisfy her needs. But me being creative wasn't enough. I wasn't a stranger who she could gradually discover, like a partner were a blooming holiday destination. No matter what lengths I went to I would always be the same resort she'd been to every year for the past decade.
I told her no. She too became sullen, passive-aggressive and emotionally abusive. Eventually, she went ahead and cheated anyway. She barely tried to hide it. When I found out, she suggested having a trial separation - basically carte blanche to have the legitimised sex she wanted. She made it plain I had three options:
a) Give her the open relationship she wanted - even if the guy she had her eye and was chasing was in a monogamous relationship himself.
b) Turn a blind eye to her cheating
c) Divorce her if I wasn't happy with any of the above.
She didn't even have the moral backbone to tell me she wasn't happy and end the relationship herself.
So I said lets cut to the chase and get divorced. By this point her behaviour was ruining my sanity and having an impact on the kids (eldest in particular was having behavioural problems at school).
I think she celebrated tbh. The whole think broke my heart - not because we split up, weirdly enough, but that it was for such a tawdry reason and the way she went about it. It cheapened everything I thought we'd had.