All I know it is one of the few things that can instantly make a person no allowed to serve a mission.
If you love a girl, you don't need to live with her to really find out. Also, it puts you at risk of breaking the law of chastity and believe me, you don't want to do that. Don't question the rules, just follow them and be happy.
Besides, it's one of the benefits of getting married, you get to live with the woman you love. Why take away one of the benefits and make the union of marriage less special?
From one guy to another, don't do it. The risks are not worth it, trust me.