Do you all have some strange idea that practicing polyamory means that all three of you must be sexually involved with each other? Even though you and she are straight? Are you aware that it is still polyamory if you are involved only with the husband? Poly people do have separate relationships - it isn't all about threesomes and moresomes. In fact, many poly people never have threesomes and have no desire to do so.
You say she is not a control freak, but you are not really "hearing" what we're telling you. The fact that she is stating any sex with her husband must take place with her present is the main reason we say she is only trying to exert control over your relationship with him. Essentially, no matter how politely and gently she puts it, that is an attempt to manage a relationship she is not in, and she has no business trying to manage. It is obvious she is insecure and not really consenting to poly if she will ask or expect this of you.
Even if you weren't heterosexual, why would you even consider it? It is extremely inappropriate for her to put that sort of requirement on his and your potential sex life. Do you get it now????