From Longman Dictionary of Contemporary Englishbe just (good) friendsbe just (good) friendsSEX/HAVE SEX WITHused to say that you are not having a romantic relationship with someone I’m not going out with Nathan – we’re just good friends. → friend
Examples from the Corpus
be just (good) friends• ""Are you going out with Liam?'' ""No, we're just good friends.''• I'm not going out with Nathan, you know - we're just friends.• Billy and I were just good friends, really good mates.• I keep telling my mother that Peter and I are just friends but she doesn't seem to believe me.• My wife and I are just good friends.• But maybe he and Jane were just friends.• They were just friends, and he was fun to be with.• Maureen and I - we thought we were just friends. 