Spoiler (mouse over to view)
Both the leads (Shoutarou and Natsumi) get divorced and then fall in love with each other. Natsumi had a shitty marriage from day 1 because her husband constantly cheated. Shoutarou's dumb bitch of an ex wife decided she didn't want to be a mother and wife anymore so she just abandons them and later on you find out she has a new kid who she will probably abandon as well.
They don't have to be married. What I liked about Orange Roof is that it shows that the 2 had to both had to go through shitty relationships & divorces to find someone who they were better with.