Why you are getting turned on is because you are seeing a nude girl in front of you while others are watching but cant touch.
Is she doing this with her own consent or you are forcing her? If she is willingly doing, then what makes you feel guilty? You should feel bad when you are at fault, from what I read, seems she is an exhibitionist, and loves to show herself to others, you are just a physically present guy with both voyure tendency to watch and exhibition tendency to show. As a man, its nothing abnormal, many has this in mind.
As you asked for suggestion, I will suggest you to get married to her. Dont just keep her shw\owing to others and dump her later. If you feel guilty, thats a good sign, but you cannot stop her, thats real bad. Soon you get married, you will see sharing her nude wont be that easy, specially when your friends and families would say, hay Adi, saw your wife's pussy, its nice and puffy and cute but dark like Sunny Leone's, you sure would come out of this habit.