1) Agree.

2) Sleep with other girls, still sleep with her.

3) She catches you, breaks up with you and bad mouths you to your friends. 

4) You: "No, we were in an open relationship. She slept with other people, too." 

Friend: "She said she was allowed to sleep with other guys, but you couldn't sleep with other girls." 

You: "WTF? Why would I possibly agree to that?" 
