Again, normally I"m a lurker but this is a personal issue for me. When I moved in with my fiance 3 yrs ago, he considered himself LDS, albeit an inactive one (he had many issues and concerns with the church at the time, which an entirely different story). I am not Mormon. His entire family on his mother's side is Catholic (though she was baptized LDS a few months ago) and his dad's side is entirely LDS. My family is Catholic. Our families addressed their concerns and advice for us, but knew that ultimately we needed to make the decision for ourselves.
I told him many times that if he wanted to go back to church or become active again I would be completely supportive of him, but that I myself would not go to church with him. We were always understanding and open. Openness is the most important thing in interfaith relationships. However, three years later my fiance no longer considers himself LDS and does not associate with the church at all. I love him and would have stayed with him whether he were LDS or not. You will have to decide how you will react if your fiance decides to become active again. Can you deal with the differences? Can you talk about the issues that come up? Can you handle church and familial pressures? These kinds of relationships can and do work. Afterall, my mother was LDS when she married my father (Catholic), my fiance's mother was Catholic when she married his father (LDS). Understanding, openness, and willingness to learn and understand is what it takes.
Just my two cents. *goes back to lurking*