My girlfriend compares me to her best male friend all of the time !?

My girlfriend has a male friend. They seem to get along very well. What I find really annoying is that she is always criticizing me and comparing me to her friend.

She always says I should be funnier like he is, that I should be slimmer like he is, that I should sing like he does singing -and he sounds horrible- and that I should be more romantic like he is !

Why does she do this? Do you think she might be cheating on me with him? She always says they are "just friends" But this behavior of hers seems really suspicious. How should I respond to this annoying behavior?
My girlfriend compares me to her best male friend all of the time !?
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