The pro-divorce movement is saying that the number of cohabiting couples will rise in the absence of divorce. It is not saying that this number will also rise if divorce is available.
So, things being equal, the balance of truth will weigh in favour of the indissolubility of marriage, since the basic difference between marriage and cohabitation lies in the presence or otherwise of the lifelong marriage bond which divorce will do away with.
Thus divorce will render allmarriages cohabitations.