Although the society is open now, there are still quite a few social groups who can't accept cohabitation. After men and women live together, they are easily pointed at by the public and labeled as "morally corrupt" by others, which increases the psychological burden of cohabitants. If you choose to live together behind your back, it may also lead to family conflicts when the truth comes out.
Many people who live together have the psychology of "playing with the opposite sex", which is unfair to each other. Moreover, after cohabitation, some people will gradually lose interest in the object and marriage because of satisfying sex, thus undermining the development of feelings.