If I understood your post correctly, I would bet that many witnesses marry only because they want to have sex (forbidden fruit), therefore it is not based on anything "real" and will eventually turn into something where people are just stuck with one another after growing separate over the years.
Humans are very much sexual beings, some much more than others and I very well understand the marry fast to have sex point of view within the cult on an individual level. On the other hand from cult's perspective using the Bible to force these more sexual humans to marry is excellent way to control them emotionally. If you follow the marriage code and happen to choose a bad mate, you are stuck and therefore their mental health eventually will decrease and it is easier to sell the "end is near, outside you will feel worse" agenda. If on the other hand you have sex before marriage, the cult will determine your faith, kick you out and you will lose everything. Either way, the couple who made bad choice loses.
However, I say that there are also many of those who have made very good choice and are happy in that sense.