While I think it would be difficult for the gf to focus in both of you at the beginning, it might change down the road. I mean think if you were suddenly faced with two brand new relationships. How would you have the energy and focus for both.
So in communication, you might want to let the gf off the hook. Say something along the lines, "It would be impossible for me to focus on two brand new relationships, so while I would like to see if we can develop something on down the line (after all, you don't want her to feel rejected either), lets put you and I on the back burner.
As for your wife, the difference between her actions and her words are the difference between her emotions and her logic. She is caught up with her new relationship, and yet everything she believes says she needs to offer you the same opportunity that she has with the gf. She is trying to be fair; she probably truly wants to be able for you to share in the experience. And she probably keeps repeating herself because she feels guilty its not going down that way. So in communicating with her, you might try to let her off the hook too. "Look I know you love me, and you want me to be able to share in your joy, but it is not really working out that way, and you don't have to feel guilty about it."
When you let people off the hook, you are more likely to get honest answers. But then you need to be honest too. Tell your wife that you supprt the relationship between her and her gf, but that sex is your issue, and that you need some quality time with just her.