Just a piece of advice: if you are going to do something (with others), do it in a way that interferes the least with the day-to-day routing you and your mono partner have. If there are times when you are alone and she is busy - use these times. Don't make her feel like sitting alone and being neglected. Be very thoughtful about making feel loved and important to you.
If you start seeing others on your "own" time, without taking away what you and your partner have as "we time," chances are that it will be easier on her and she will trust you even more.