The main thing is, we do want to follow our faith (something we both chose) and honor our commitment to abstain till marriage (yeah, folks, it's doable but insanely hard as is). Living together would make it harder, even if we were sleeping in separate rooms. Just think about it, even if you're not in the same bedroom, it's still just the two of you, seeing each other get out of the shower, in your PJ's, lots of opportunities to snuggle on the couch......come on, if none of that tempts you, you're probably dead. Waiting is already hard and I've come close to falling several times. Why make it harder?
Faith and traditional morals aside, what if, after all that, it didn't end up working? Breaking up is tough for many couples, can you imagine paying money to break the lease, as well as needing to cough up first/last month's rent and a security deposit, as well as a lease fee and an application fee? Not to mention the stress of moving your stuff out. I know moving already stresses me out. I can't imagine this scenario. I've known both married (but obviously separated) and unmarried couples who've gone through this. Not for me.
Also, if I were to live with someone, without making a serious commitment, to me, that would put the idea of, "well, one of us could just leave," in my subconscious. To me, if I am going to move in with someone, share my daily life with him and he with me, there's no turning back. If you're moving in with me, then you better be prepared to stand with me before God, the law, and all of our family and friends and make a commitment that binds you till the day one of us dies. Because, by the point I'm ready to live with you, I've already decided to love you till the day I die.