One WG I dated years ago, I banged her more as a customer than when I was her bf, mainly because she was always working. Also she looked way hotter at work all dolled up, whereas when I became her bf she was more plain around me. If your goal is just to have fun with a girl, don't convert her to a relationship, a relationship has to be about more than just fun in the bed.
Also when you look at how people can change within 5 years, time catches up to everyone, so your gf will let shit slide. Being a customer is better because new girls come and go and they stay the same age.
No matter how hot a girl is, you will get bored fucking the same girl long term.