YES they can stay together if they know what their goal are and practice save sex (if they are intrested) then their nothing to debate about.They are their own bosses of their life , they should know the rises and timeout of relationships and the consequences if it goes not as expected.
I cannot answer yes or no to this question. In today's world college students have co-ed room mates. Others have room mates to bring down bills or other reasons. Sometimes it is good for a person to live with the other before deciding to get married in order to see if the cohabitation will work. Many times people jump into marriage and find that something about their mate, they are not willing to deal with.
Those who live together before they are married lose respect for one another. When you don't have mutual respect, the relationship fails. There doesn't need to be any other reason than this. I suppose one could argue that, but I am pretty sure we can agree that respect for one another ranks up there as one of the most important aspects for a relationship to work.
For me, I would not want my sister to live with somebody without getting married, praise be to Allah, she is not like that girls who want to live.
If I had a sister like that I would deny being his brother.
However, If I live with a gril without getting married she is also a sister of somebody else.