Is your girlfriend very body confident? I'm very happy with my body and feel comfortable naked, and being naked with my partner, so wouldn't have a problem with this. But I can imagine anyone, male or female, or isn't totally au fait with all of themselves might feel self-conscious.
I would only have sex with someone who I felt completely understood me and/or respected me, so doing this in a romantic/sexual/kinky way would not be an issue, for me.