Do you like the concept of close friends sharing the same flat even after marriage?
my personal answer to this question is an outright No and a very big no with capital letters NO.
when one is married, he and his wife are meant to live alone and not with others at least for the first few formative years of their marriage where they go through the bonding stage, the child bearing stage et all.
the presence of other might spell doom to the marriage and it doesn't really make sense for married couples to share flats because they would definitely encounter problems from living with others.
furthermore, i personally do not subscribe to sharing a flat for married people considering the fact that they would bear children and they would still be sharing same flat while raising their kids, this is tantamount to make the child upbringing experience extremely stressful.
also, being a bachelor i personally do not like sharing a flat with others for the sake of privacy and this factor is another reason why married couples should share flats with others because there won't be privacy. you cannot even say someone is invading your privacy in this case because there is no privacy in the first place.