It is a fresh experience when I became a second-graded student, two of my roommates started to stay overnight in their boyfriends' houses. At first, I thought it was really a big step forward to the relationship and really didn't approve it. However, I soon found out that this might be a great start for two people to understand each other more because they could get used to each other's living habits earlier, so I turned to persuade my roommates to move to live with their boyfriends next semester. It was reported many times that lots of couples figured out they didn't fit when they finally built a dreamy sweet house.
To my surprise, they all refused. They told me they couldn't accept premarital cohabitation or something like that because they insisted on the sense of fresh was important than their future happiness. So I asked deeply and discovered that they didn't stand against premarital cohabitation but they fought against "cohabitation before engagement". The definition of these two terms was a little different that the range of the second one term was much smaller than the first one. They said when somebody came up the thought of marrying someone else , it was necessary to live together for a while to make a final decision; however, when two people hadn't thought to make a forever contract to each other, they shouldn't live together hastily.
It was unexpected because I think the thing in another way. I support any kind of cohabitation since there is no difference between temporary staying or long-time-stay. Discontinuous living together is actually a reflection of the desire to live together. If you want to stay longer with your lover and you have ability to do, why not just moving into his house?