Would it help if you assured her that when the model was on the bed, you were not: you were behind the camera?
People attach different meanings to objects, and you may never be able to take your girlfriend's point of view about bed symbolism or make her understand yours. If you ever move in together, you may find conflicts arising because a household object has sentimental value for you or her (e.g., a family heirloom) but the other one doesn't like its looks or its functionality. I think the important thing is to remain respectful when discussing such differences.
I tried to explain that! I even went in detail regarding how many different things a photographer is considering while he/she is taking even a single photograph. Like you said, it is a symbolic thing for her and I feel like there will be little budging. I can see what you mean about once we move in together -- I respect her wishes and I told her I wouldn't actively seek to do Photoshoots at my own apartment (by virtue, I treat it as a last option in terms of location) but that I wouldn't not-do-it if it came to that. Some sort of compromise, anyway..