Moving in with significant other. Advantages & Disadvantages?

My girlfriend and myself are looking to move in with each other. Her parents are very much against this as they are very traditional and don't believe two people should be living with each other until their wedding day. This is not going to work for us. What are some of the benefits and some of the disadvantages to living with your significant other? I want to be prepared for this lengthy discussion I'm sure I will be having with the parents.