Why bride and groom families sit separate in church?

Most people attending a wedding only know one of the couple. It is natural for them to sit on that side and with their relations and friends that they know, rather than sitting in amongst strangers. It is also natural for the immediate family members to sit together on the side of their family member that is getting married. Behind them will be their relations and friends. If you were going to a wedding of a family member or relation, you would naturally sit with your family. If you were not part of the immediate family and only knew one of the couple, you are more likely to sit with friends or relations of yours, who are more likely to be sitting on the side of the person that you know. It can also be symbolic, in that the couple come together as two separate individuals and so the people that know them sit separately, like they often light two separate candles before the ceremony and then light one candle together after they are married to symbolise they are now one. Later in the day there will be plenty of opportunity for the people to mix.