I really appreciate the advice. My wife and I have discussed this at length. Her primary concern seems to be a fear of lost intimacy, and she feels a little threatened by our girlfriend. Its nothing that our girlfriend has done; its just that she's younger (23) and attractive. My wife is only 29 and she is traffic-stopping beautiful, but she's still insecure where I'm concerned. Also, our GF practically lives with us. While she enjoys the time they share, my wife misses the time her and I spent alone and would like to schedule more one-on-one time for us. I've agreed to set that time aside and be aware of her needs when I schedule outings.
My wife promises to be patient, work through her insecurities, and to consider the needs / expectations of our girlfriend. She is deeply bisexual and is much happier when she has both a man and a woman in her life. While the sex is amazing, I like to believe it's more than that; we are all invested in seeing this become a positive and enriching influence in our lives.
Thanks again for taking the time to contribute.