Welcome vat152 to JWD - I will try to answer you but it sounds like you have a very complicated situation and there could be a number of reasons why your fiance may have this problem with the intimate side of your relationship. What you probably don't realise is that the JW's is a high mind control religion, in other words it is a cult.
Although your fiance was disfellowshipped and is living with you now, you say that she still defends/believes in their teachings and that you suspect she may want to go back. If this is the case then she has many conflicting thoughts and emotions over her relationship with you at the moment because for her to be reinstated as a JW she could not be having a relationship with you or live with you without being married. Also she knows that to be a JW and married to someone who is not a JW is not approved of and would cause very serious problems anyway because of the way of life a JW has to live.
I don't know how much you know about the JW's but I suggest you do some research to know what you are up against. It would also help your fiance to know the truth about the Organisation as it may help free her from the mind control. There is a great deal of information available on the internet now which has helped many of us on JWD, and keep posting here for support and information.
I hope this helps and I wish you well
Maddie