While I don't have that kind of rule in place for myself. I can understand it. If thats something she needs then I don't know if it is that hard.
In any V... there are 4 relationships. Someone above mentioned getting to know her better. Maybe thats the key. If you can build up the friendship she wouldn't just have trust in her husband, but in you too. There is a deep intimacy when sleep with someone. I can't just "sleep" with anyone in the bed. Maybe she feels that way as well.
Being open... is easy. Being poly can be hard work. Fucking for some people is an easier thing to accept than love/intimacy.