I'm of the mindset that most people who have kids are, which is, 'Hey, I want another me. I like me. I'm pretty cool, and I've got really great ideas, and the way I think is the right way to think. Let's put another one of me out there.' So I'll have kids one day.
As for Tenacious D, of course it could work as a full length movie; all it requires is a great writer and great director with an ability to think outside of conventional film comedy.
We mostly get together and have fun. It's a great place to play music with a small group of nice people.