Now, I am not trying to start an arguement, just trying to figure this out, but how is this different from cohabiting if they aren’t validly married? I mean, they are told to continue to live together but remain chaste. It would seem to me that if cohabitation is wrong because the couple isn’t validly married yet, then the same should apply to couples who aren’t validly married in the church.
I understand. I am not a pastor, so I can’t reliably explain why they do what they do. I can observe, though.

When a couple who is invalidly married comes round to get the marriage convalidated, they tell them to abstain. They don’t seem to tell them to move out for several months. I wonder if this is partly related to how the convalidation is often a private event. Myself, if I were to suddenly find my marriage were invalid for some unknown reason, I’d not wish to advertise this to the world. I’d want it remedied very quietly. This would cause the least amount of scandal within my family. If it were public, it would cause lots of scoffing at the Church and its “dumb rules” and “lunacy”. For my relatives sake, it would be better if they were to never know. And they would know if we were separated for several months. (For clarity, I was married in the Church with said relatives in attendance). What I’m saying is perhaps the pastor makes an assessment of potential scandal. For other people, perhaps less scandal would be caused if they were forced to separate and the convalidation was very public.
If a couple who was Protestant and had married according to Protestant rules were to come to the pastor to convert to Catholicism, they would not be told that they had to abstain. Their marriage would be considered valid. (Let’s assume this was their first marriage).
It would seem that the church should advise them (an invalidly married couple, waiting for church blessing) to live apart until they are validly married. I think they don’t because they realize that to live apart one person has to find a new place (usually a rental) and that takes time, and depending on the location, is nearly impossible (I am thinking of my home town where there simply are no cheap rentals)
I will add to your thoughts here that often one is going to be responsible for an entire year’s lease, and not just a few months. The relatively cheap rentals in my area consist of an 8 foot by 8 foot cubicle, with a unisex shared bathroom down the hall and no kitchen facilities. Over ten years ago, such a sleeping room cost $325 a month (uh, more likely $425, my mind is failing). It would be more now.
And in the case of this hypothetical couple people are talking about, what is so wrong with separate bedrooms and living like “room mates?”
I could never have lived with my future spouse as I would with a roommate. The relationship was irrevocably different. I’m not talking about sexual relations. I’m talking about everything else though, including touch.
Also a term that needs to be better defined is what exactly is cohabitation? Some said I could be guilty of that because for 3 months I moved all my stuff into my fiancee’s apartment because I had no place for it. I ate his food, used his kitchen and stayed with him until late at night. Then, before we fell asleep I would get up and go to my friend’s house to sleep in a separate bed. (for the 3 months before our wedding after graduation) Most people i know would say this is NOT cohabitation. So what about a couple who has all their stuff in the same house, and have separate bedrooms? How far apart do the bedrooms have to be for “cohabitation” not to be the term?
Myself, looking at how my relationship with my future spouse grew, I’d say there is only a small gap between what you describe and with living together, at least when considering taking a couple that is currently living together and splitting them up for a bit. The interactions between the couple are the same. The way the pass the sugar is the same.
What that gap consists of exactly…hmmm. This is an excellent question to consider. I don’t think it is “commitment”, since they are marrying in a few months. It could be “respect”, at least if one of the two wants to be separate for the few months. I don’t know. That you ask the question almost makes me want to suggest that in general it would be better not to have the lives so intertwined as you described at any point before marriage. Mine was, though, so I have no grounds to talk, other than to say I wish it hadn’t been that way. It would have been better if at no point in our relationship had our lives been so intertwined. Temporary separation made no difference. (We weren’t separated by the Church or anything, just we were on opposite sides of the country for employment reasons for 4 months and couldn’t afford to visit, only a little phone and lots of writing).