sparky this shouldn't be a dilema, but since its seems to be, i geuss my opinion will be of value.
ok, go have a serious discussion with your inner self. do u genuenly feel her, would u want to spend the rest of your life with her?
if yes to these, then take her for an exclusive dinner.
begin a serious talk and explain to her why you feel, that if you have to spend the next 40yrs + together then you need to follow some guidelines. you can tell her you love her so much n yu wanna marry her officially before moving in with her....point is don't hurt her feelings at all cost. be as candind and respectful as you possibly can be.
best of luck.
remember to talk in a respectful manner.