Daddy/daughter play isn't necessarily an incest fantasy.

Think father figure/young girl who needs guidance. That can manifest itself in many ways. Guardian/ward, headmaster/student, even governess/youthful charge.

A very hot pseudo-incest version would be two fathers who swap their hot daughters into each others' care.

If you're worried about the girl friend's reaction, any of the above could make it palatable to her and you can be imagining anything you want while playing.