Polyamory provides us the opportunity to express different aspect of our personality with different people. With the girlfriend, I tend to be more submissive, whereas the wife is completely turned off with anything pertaining to D/s. Sometimes it is easy to explore something new with somebody new.
In your situation, things are trickier. I know I have been envious of guys doing something with my wife that I would also enjoy doing. I have lost count of the number of times I have caught myself asking the question, "why doesn't she do that with me?" But for us, it doesn't work that way.
Unfortunately, I have no easy answer. I acknowledge that my wife is free to pursue her relationships independently of me. I don't bar her from doing things with others, nor do I demand that they be done with me. She get to decide. Period.
As for me, I understand and strongly believe in reasoning behind this freedom of choice, even though I may not like how it plays out.