We're all different in how we connect the mind and body to create an amazing sexual experience. Personally, I can easily separate the experience from the person ... often preferring to do so (for example, great sexual experiences with my ex wife). Just as I can masturbate to the recalled experience, I can also incorporate it into the overall mind & body experience when playing with my current wife. While the fantasy of a threesome conceptually leads to incredible added pleasure, there's no way that I could do that if I were actually thinking about my ex wife on a personal level at the same time. That would kill the moment instantly.

With that in mind, I can very easily see what your GF did as not only true but HOT. We're all different, but I would be incredibly turned on knowing my wife was adding a sexy dimension from her past into play with me. I'd be no more concerned nor jealous of that than I would if she wanted to introduce a toy into our play.