Well, it's true. It's entirely possible to end up falling for someone after living with them for so long. I guess you sort of form a special bond (by trusting each other and etc). However, I don't know if what they would feel for each other is as true a love as say another couple who fell in love before they decided on marriage.
We can all talk the talk but unless we somehow manage to try both, I guess we'll never know the truth. And maybe it's for the better. So whatever you might end up with, you can convince yourself that it's the better option