Sedan and coupe are terms that go all the way back to coachbuilding. If you look at cars from the 40s and earlier, you'll see that coupes and sedans were completely different body styles. There were two door sedans and four door sedans, but coupes were always two doors and for a long time they were only two seaters. Over the years coupe and sedan have just come to mean how many doors a car has. If you want to get really technical, a car like the Thunderbird should probably be a two-door sedan because it has a backseat...but no one today uses the terms as they were originally meant, so it doesn't really matter.
Executive Editor, Ford Builder Magazine--hit me up if your ride is worthy!