Stop the focus on her. Focus on him, and what he is doing. It is responsability to convey to anyone he is with what HE, as an adult, has decided not to do.
Talk to you husband. Get him to initiate some kind of contact between the two of you. Metamours should be able to at least talk to each other. also, communication is so much easier when your husband does not have to to all this message-carrrying.
Go to at least one sex party, just as someone watching. See for yourself what it is like. You can take your husband with you if you like, or someone else you trust. Be curious; it you don't like it, why? This may help you with your arguments should you contiue to veto him not freqnenting these parties.
Consider how your husband's girlfriend can be included in your life. Is that what she wants? Or does she want more time with him alone? How can it be arranged? What are you willing to do to open up your heart and home to her - and vice verca?