Originally Posted by quean
Just a thought, but maybe your wife would like to join in but can't admit this to you or herself?
The OP says that his wife is monogamous and struggling with the situation whenever he goes out on dates with the gf. I highly doubt that she would want to participate or watch.
Nudger, I would suspect that her discomfort might be related to insecurities, which are usually underneath any jealousy people feel. She probably took your need to have an additional partner as a blow to her self-esteem and questions her value and worth. She is mono, she doesn't understand how you can want someone else when you are all she wants.
The best thing you can do is reassure her, and make sure you are as loving with her as you are with the gf. Do you romance your wife? Make an effort to let her know she is not insufficient in any way? Make sure that the time you spend with your wife isn't only relegated to household/bill-paying/family stuff, while the girlfriend is all about hot sexy times. I say take your wife out on dates and invite the girlfriend over to fold laundry.
There are also some email lists/groups for people in poly/mono relationships. She might find some good support here, which is for the monos: http://groups.yahoo.com/group/PolyMono/