I am against the cohabitation before marriage. Firstly, I can give dozens of examples when young couples break up their relationships after several months even years of living together. Secondly, I think that especially a girl should live only with her husband.
You say you can give dozens of examples of young couples breaking up after living together. Well that's a huge shocker that some couples broke up after living together. Cohabitation before marriage is one of the stepping stones to knowing if you should get married. If you don't live together but have been together for awhile and then get married and then move in together then you learn about each others bad habits it will put an instant strain on the marriage. Also a girl is free to live with whoever she wants to live with same with anybody, it's called free will. If you live with your boyfriend or girlfriend before you get married you will learn about each others bad habits you will start getting more in sync with each other and you really get to know each other. Not to mention I can also give a whole lot of examples where couples have lived together first and then got married and everything has been fine.
If they could not get accustomed to their Habits, if they broke their relationships, they should try with every partner until they find thier real partners? Also, may be one of the partners is happy just living together, without marriage. How the second partner should solve this problem? Also it takes experiment after getting marriage. And the last point, the couple might have religious problems, for instance, if one of them is not going to have sex before marriage. Some religions are very strict about it.
If they wish to move in with every partner they have people are more than welcome to do that. Also just because you marry someone doesn't always mean they are your real partner as you say. Everyone is different. But to live together before you get married is a key step in any relationship and will see if you can actually stand living with each other. I have heard many stories of people breaking up after they have moved in together because they couldn't stand living with each other and realized they didn't actually work out together, imagine if they had gotten married first. The second partner doesn't have to solve this problem if in the end they are both happy, you don't have to get married to prove you love someone nor do you have to get married just to live with the other person. Also sex before marriage if that is a real issue for a couple then they are quite likely not truly meant to be with each other and will break up or they will get married if they truly love each other. No guy will marry a woman just to have sex with her. Couples have the right to move in together before marriage and in the end it will always be the better choice than not living together then jumping straight into marriage.
Have you ever thought about psychological sides of this? When you get married first, you take responsibility. But when it is just cohabitation, a person understands that if they cannot live together, they just can break up. Moreover, a lot of couples get married without cohabitation and getting used to each other and live with happy life. Cohabitation is just pretext to live together without any responsibility.
So rushing into marriage without living together moving in together than realizing you can't stand each other after awhile but want to give it a shot because you are married thus making the relationship harder which leads to one or both parties becoming unfaithful or having arguments quite often and eventually getting divorced, is that the psychological stuff you are talking about? Cohabitation does not mean you are entering it think oh well if it doesn't work out we'll break up, moving into together is a choice you have made with your partner because you both think you are ready to take the next step in the relationship. Neither party ever thinks ok if this doesn't work out we'll just break up. A lot will stay together even if there are problems because they choose to want to work on it rather than feeling forced to have to work on it because they got married to soon. Moving in together actually requires a lot of responsibility. Also a lot more couples get married after cohabitation together rather than before. Also you are saying that the couples get used to each other living together when they are married which is the exact same thing as moving in together before they get married they feel each other out start getting use to each other around all the time and finding out each others bad habits and then moving past them because they have made that choice because they want it to work rather than feeling forced they have to make it work for the sake of a marriage. In conclusion I am for couples living together before marriage it is a very natural thing for couples to do and the relationship ends up better for it no matter the outcome.