I've probably written about this before, but one of the main appeals of Futurama is that the protagonists are not a family.
Disagree. The cast dynamic is clearly family-oriented.
Families always end up loving and caring about one another, and with one notable exception the members can not screw.
While there's not a whole lot of screwing going on between members of the Planet Express crew, there's a lot of screwing going on between them and extraneous characters. So I'll grant that in contrast with, say, The Simpsons, where the kids are preteens.
I don't think it holds water across the board for a family dynamic, though; it's a question of the characters' ages. One of the funnier eps of American Dad, IMO, was where Hayley was screwing Bullock. Hell, even Meg's gotten laid a couple of times.
Furthermore, you have a harder time aging your characters, because kids grow up and move out and then what happens to your show? And let's face it, kids aren't as fun to watch as adults. Adults can do anything. Kids can't do squat.
Characters with different levels of maturity are a classic dynamic in comedy. Age and maturity aren't the same thing, but they're related. Fry's as close to a kid as you get on Futurama (the occasional Cubert/Dwight episode notwithstanding, and they're STILL more mature than he is in a lot of ways -- the "kids who are more adult than the adults" angle is another comedy staple; see also Stewie and Lisa), and that's essential to his character.
