She doesn't sound all that poly to me. Try saying the exact same things she told you with the exact same tone. Wait your not seeing anybody yet....
Do you think it might be possible that she's using this poly flag so she won't get hurt by another guy.....not putting all eggs in one basket.... I think some people don't want be that vulnerable again with one person. Maybe that's why you were just the guy see was seeing....her heart has been smashed too many times before. You might be paying some other guys sins.
How does she say these things with a straight face....and why is it ok for her and not for you?