I don't know if they are my favorite fictional couple or not, but they are the first ones that popped into my mind.
In the 1968 movie, "Yours, Mine, and Ours", you follow Henry Fonda and Lucille Ball as they try to blend their families. Here is the blurb:
"When a widower with 10 children marries a widow with 8, can the 20 of them ever come together as one big happy family? From finding a house big enough for all of them and learning to make 18 school lunches, to coping with a son going off to war and an unexpected addition to the family, Yours, Mine and Ours attempts to blend two families into one and hopes to answer the question Is bigger really better? "
I like the movie a lot and have watched it many times. This is a sweet romance, and one that really shows love between two people. They even try to not see each other because it would be too hard on their children to adjust.
It's a good movie if you've never seen it. I do not like the remake.