You may already know this part: What you describe is a vee relationship, with yourself as the "hinge" and your wife as one side of the vee and your GF as the other. Your wife and you are the "primary" relationship, you and your GF are in a "secondary" relationship. (Some people object to these labels, but for now they can be used as a kind of surface description.)
I suppose the first thing you might consider is having your wife meet your GF. It seems that when the primary partner knows and, hopefully, is friendly with the secondary partner then many of the fears you describe evaporate. Friendship and understanding between all partners is a big, BIG plus.
You also might consider some general rules and boundaries. Again, some people dislike these but it's something to think about especially at the beginning. Some examples: you and your GF might agree not to sleep overnight with each other. This reassures your wife because she is the only one who spends a full night in your bed. You might agree to see your GF only twice a week, say, and also agree to special weekly date nights with your wife.
Boundaries can be reassuring in a new situation.
The other thing is of course to keep communicating about what is going on. Certainly if you talk constantly about your GF then your wife will start to resent the relationship. But she needs to know enough to feel secure and safe in her own relationship with you, and so that means she needs to know what you are thinking with regards to your marriage and your other friendships.
Last edited by EugenePoet; 04-30-2010 at 01:10 AM.