In my opinion, sharing a residence with someone of the opposite sex really depends on why you've chosen to do so. If it's romantic, then my personal belief is people should get married first; otherwise, they're just setting themselves up for tragedy.
I speak from experience. I lived with an ex-GF for 6 months in Georgia; that situation ended when she decided not to commit, even though she said she loved me. I wound up going back home to Texas (with a severely broken heart), and in the last 15 months since, I've gained a whole new perspective on the risks involved with arrangements like that. No offense to those for whom it actually (and rarely) works long-term, but in my opinion, it's like playing with a sharp knife and expecting to remain unhurt. Love and lifelong commitment really need to coexist; otherwise, it just defeats the point.