I think this girl is bring very immature and clearly does not deserve you, any sort of sexual contact is cheating, and to be honest shes only 16 and she getting it 'on' with different guys??? Do you really want to be in a relationship where your partner doesnt respect you?
It does not matter of her friends or your thinks its not cheating, your in the relationship not them and if you consider it cheating thats enough...
Ive been young before (Im only 22 lol) and at that age a relationship feels like its everything and that you cant imagin seein urself with no one else... But let me tell you something, there is better than her out there, who will love you and respect you above all...
Don't fight a pointless battle you two clearly don't share the same values... its not gonna work...