The way you describe her as average and thin, how you are being retained by your ex gf and how you accepted it just shows whats really in your mind . You felt you are too young to be confine to a girl only and a better girl could be next.

If she only wants sex,any physically good guy will do. Either you stay with her and just leave it all behind without the guilt on your part and her clinging on to you.

It takes a true gentleman to love and a boy to love many.